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Original authorsKeir Fraser, Steven Hand, Ian Pratt, University of Cambridge Computer Laboratory
DevelopersLinux Foundation
Intel
Initial releaseOctober 2, 2003; 22 years ago (2003-10-02)[1][2]
Stable release
4.20[3] Edit this on Wikidata / 6 March 2025; 7 months ago (6 March 2025)
Repository
Written inC
Platform
  • x86
  • ARM
  • RISC-V
  • PowerPC
TypeHypervisor
LicenseGPLv2
Websitexenproject.org

Xen (pronounced /ˈzɛn/) is a free and open-source type-1 hypervisor, providing services that allow multiple computer operating systems to execute on the same computer hardware concurrently. It was originally developed by the University of Cambridge Computer Laboratory and is now being developed by the Linux Foundation with support from Intel, Citrix, Arm Ltd, Huawei, AWS, Alibaba Cloud, AMD, Bitdefender and EPAM Systems.

The Xen Project community develops and maintains Xen Project as free and open-source software, subject to the requirements of the GNU General Public License (GPL), version 2. Xen Project is currently available for the IA-32, x86-64 and ARM instruction sets.[4]

Software architecture

[edit]

Xen Project runs in a more privileged CPU state than any other software on the machine, except for firmware.

Responsibilities of the hypervisor include memory management and CPU scheduling of all virtual machines ("domains"), and for launching the most privileged domain ("dom0") - the only virtual machine which by default has direct access to hardware. From the dom0 the hypervisor can be managed and unprivileged domains ("domU") can be launched.[5]

The dom0 domain is typically a version of Linux or BSD. User domains may either be traditional operating systems, such as Microsoft Windows under which privileged instructions are provided by hardware virtualization instructions (if the host processor supports x86 virtualization, e.g., Intel VT-x and AMD-V),[6] or paravirtualized operating systems whereby the operating system is aware that it is running inside a virtual machine, and so makes hypercalls directly, rather than issuing privileged instructions.

Xen Project boots from a bootloader such as GNU GRUB, and then usually loads a paravirtualized host operating system into the host domain (dom0).

History

[edit]

Xen originated as a research project at the University of Cambridge led by Ian Pratt, a senior lecturer in the Computer Laboratory, and his PhD student Keir Fraser. According to Anil Madhavapeddy, an early contributor, Xen started as a bet on whether Fraser could make multiple Linux Kernels boot on the same hardware in a weekend.[7] The first public release of Xen was made in 2003, with v1.0 following in 2004. Soon after, Pratt and Fraser along with other Cambridge alumni including Simon Crosby and founding CEO Nick Gault created XenSource Inc. to turn Xen into a competitive enterprise product.

To support embedded systems such as smartphone/ IoT with relatively scarce hardware computing resources, the Secure Xen ARM architecture on an ARM CPU was exhibited at Xen Summit on April 17, 2007, held in IBM TJ Watson.[8][9] The first public release of Secure Xen ARM source code was made at Xen Summit on June 24, 2008[10][11] by Sang-bum Suh,[12] a Cambridge alumnus, in Samsung Electronics.

On October 22, 2007, Citrix Systems completed its acquisition of XenSource,[13] and the Xen Project moved to the xen.org domain. This move had started some time previously, and made public the existence of the Xen Project Advisory Board (Xen AB), which had members from Citrix, IBM, Intel, Hewlett-Packard, Novell, Red Hat, Sun Microsystems and Oracle. The Xen Advisory Board advises the Xen Project leader and is responsible for the Xen trademark,[14] which Citrix has freely licensed to all vendors and projects that implement the Xen hypervisor.[15] Citrix also used the Xen brand itself for some proprietary products unrelated to Xen, including XenApp and XenDesktop.

On April 15, 2013, it was announced that the Xen Project was moved under the auspices of the Linux Foundation as a Collaborative Project.[16] The Linux Foundation launched a new trademark for "Xen Project" to differentiate the project from any commercial use of the older "Xen" trademark. A new community website was launched at xenproject.org[17] as part of the transfer. Project members at the time of the announcement included: Amazon, AMD, Bromium, CA Technologies, Calxeda, Cisco, Citrix, Google, Intel, Oracle, Samsung, and Verizon.[18] The Xen project itself is self-governing.[19]

Since version 3.0 of the Linux kernel, Xen support for dom0 and domU exists in the mainline kernel.[20]

Release history

[edit]
Version Release date Notes
1.0 October 2, 2003[1][2]
2.0 November 5, 2004[21] Live migration of PV guests.
3.0 December 5, 2005[22][23]

The releases up to 3.0.4 also added:

3.1 May 18, 2007[27] Live migration for HVM guests, XenAPI
3.2 January 17, 2008[28] PCI passthrough and ACPI S3 standby mode for the host system.
3.3 August 24, 2008[29] Improvements for the PCI passthrough and the power management. Xen ARM hypervisor source code released for ARM CPU support
3.4 May 18, 2009[30] Contains a first version of the "Xen Client Initiative", shortly XCI.
4.0 April 7, 2010[31] Makes it possible to use a dom0 Linux kernel, which has been implemented by using PVOps. A Linux kernel of version 2.6.31 has been modified for this purpose, because the official Linux kernel actually does not support the usage as dom0 kernel (date July 2010).[32]
4.1 March 25, 2011[33] Some of the improvements: Support for more than 255 processors, better stability. Linux kernel v2.6.37 and onward support usage as dom0 kernel.[34]
4.2 September 8, 2012[35] XL became the default toolstack. Support for up to 4095 host processors and up to 512 guest processors.
4.3 July 9, 2013[36] Experimental ARM support. NUMA-aware scheduling. Support for Open vSwitch.
4.4 March 10, 2014[37] Solid libvirt support for libxl, new scalable event channel interface, hypervisor ABI for ARM declared stable, Nested Virtualization on Intel hardware.[38][39]
4.5 January 17, 2015[40] With 43 major new features, 4.5 includes the most updates in the project's history.[40]
4.6 October 13, 2015[35] Focused on improving code quality, security hardening, enablement of security appliances, and release cycle predictability.[35]
4.7 June 24, 2016[41] Improved: security, live migrations, performances and workload. Hardware support (ARM and Intel Xeon).[42]
4.8.1 April 12, 2017[43]
4.9 June 28, 2017[44] Xen Project 4.9 Release Notes
4.10 December 12, 2017[45] Xen Project 4.10 Release Notes
4.11 July 10, 2018[46] Xen Project 4.11 Release Notes
4.12 April 2, 2019[47] Xen Project 4.12 Release Notes
4.13 December 18, 2019[48] Xen Project 4.13 Release Notes
4.14 July 24, 2020 Xen Project 4.14 Release Notes
4.15 April 8, 2021 Xen Project 4.15 Release Notes
4.16 December 2, 2021 Xen Project 4.16 Release Notes
4.17 December 14, 2022 Xen Project 4.17 Release Notes
4.18 November 23, 2023 Xen Project 4.18 Release Notes
4.19 July 29, 2024 Xen Project 4.19 Release Notes
4.20 March 5, 2025 Xen Project 4.20 Release Notes

Uses

[edit]

Internet hosting service companies use hypervisors to provide virtual private servers. Amazon EC2 (from August 2006 to November 2017),[49] IBM SoftLayer,[50] Liquid Web, Fujitsu Global Cloud Platform,[51] Linode, OrionVM[52] and Rackspace Cloud use Xen as the primary VM hypervisor for their product offerings.[53]

Virtual machine monitors (also known as hypervisors) also often operate on mainframes and large servers running IBM, HP, and other systems.[citation needed] Server virtualization can provide benefits such as:

  • Consolidation leading to increased utilization
  • Rapid provisioning
  • Dynamic fault tolerance against software failures (through rapid bootstrapping or rebooting)
  • Hardware fault tolerance (through migration of a virtual machine to different hardware)
  • Secure separations of virtual operating systems
  • Support for legacy software as well as new OS instances on the same computer

Xen's support for virtual machine live migration from one host to another allows load balancing and the avoidance of downtime.

Virtualization also has benefits when working on development (including the development of operating systems): running the new system as a guest avoids the need to reboot the physical computer whenever a bug occurs. Sandboxed guest systems can also help in computer-security research, allowing study of the effects of some virus or worm without the possibility of compromising the host system.

Finally, hardware appliance vendors may decide to ship their appliance running several guest systems, so as to be able to execute various pieces of software that require different operating systems. [citation needed]

Types of virtualization

[edit]

Xen offers five approaches to running the guest operating system:[54][55][56]

  • PV (paravirtualization): Virtualization-aware Guest and devices.
  • HVM (hardware virtual machine): Fully hardware-assisted virtualization with emulated devices.
  • HVM with PV drivers: Fully hardware-assisted virtualization with PV drivers for IO devices.
  • PVHVM (paravirtualization with hardware virtualization): PV supported hardware-assisted virtualization with PV drivers for IO devices.
  • PVH (PV in an HVM container): Fully paravirtualized Guest accelerated by hardware-assisted virtualization where available.

Xen provides a form of virtualization known as paravirtualization, in which guests run a modified operating system. The guests are modified to use a special hypercall ABI, instead of certain architectural features. Through paravirtualization, Xen can achieve high performance even on its host architecture (x86) which has a reputation for non-cooperation with traditional virtualization techniques.[57][58] Xen can run paravirtualized guests ("PV guests" in Xen terminology) even on CPUs without any explicit support for virtualization. Paravirtualization avoids the need to emulate a full set of hardware and firmware services, which makes a PV system simpler to manage and reduces the attack surface exposed to potentially malicious guests. On 32-bit x86, the Xen host kernel code runs in Ring 0, while the hosted domains run in Ring 1 (kernel) and Ring 3 (applications).

CPUs that support virtualization make it possible to run unmodified guests, including proprietary operating systems (such as Microsoft Windows). This is known as hardware-assisted virtualization, however, in Xen this is known as hardware virtual machine (HVM). HVM extensions provide additional execution modes, with an explicit distinction between the most-privileged modes used by the hypervisor with access to the real hardware (called "root mode" in x86) and the less-privileged modes used by guest kernels and applications with "hardware" accesses under complete control of the hypervisor (in x86, known as "non-root mode"; both root and non-root mode have Rings 0–3). Both Intel and AMD have contributed modifications to Xen to exploit their respective Intel VT-x and AMD-V architecture extensions.[59] Use of ARM v7A and v8A virtualization extensions came with Xen 4.3.[60] HVM extensions also often offer new instructions to allow direct calls by a paravirtualized guest/driver into the hypervisor, typically used for I/O or other operations needing high performance. These allow HVM guests with suitable minor modifications to gain many of the performance benefits of paravirtualized I/O. In current versions of Xen (up to 4.2) only fully virtualized HVM guests can make use of hardware facilities for multiple independent levels of memory protection and paging. As a result, for some workloads, HVM guests with PV drivers (also known as PV-on-HVM, or PVH) provide better performance than pure PV guests. Xen HVM has device emulation based on the QEMU project to provide I/O virtualization to the virtual machines. The system emulates hardware via a patched QEMU "device manager" (qemu-dm) daemon running as a backend in dom0. This means that the virtualized machines see an emulated version of a fairly basic PC. In a performance-critical environment, PV-on-HVM disk and network drivers are used during the normal guest operation, so that the emulated PC hardware is mostly used for booting.

Features

[edit]

Administrators can "live migrate" Xen virtual machines between physical hosts across a LAN without loss of availability. During this procedure, the LAN iteratively copies the memory of the virtual machine to the destination without stopping its execution. The process requires a stoppage of around 60–300 ms to perform final synchronization before the virtual machine begins executing at its final destination, providing an illusion of seamless migration. Similar technology can serve to suspend running virtual machines to disk, "freezing" their running state for resumption at a later date.

Xen can scale to 4095 physical CPUs, 256 VCPUs[clarification needed] per HVM guest, 512 VCPUs per PV guest, 16 TB of RAM per host, and up to 1 TB of RAM per HVM guest or 512 GB of RAM per PV guest.[61]

Availability

[edit]

The Xen hypervisor has been ported to a number of processor families:

  • Intel: IA-32, IA-64 (before version 4.2[62]), x86-64
  • PowerPC: previously supported under the XenPPC project, no longer active after Xen 3.2[63]
  • ARM: previously supported under the XenARM project for older versions of ARM without virtualization extensions, such as the Cortex-A9. Currently[when?] supported since Xen 4.3 for newer versions of the ARM with virtualization extensions, such as the Cortex-A15.
  • MIPS: XLP832 experimental port[64]

Hosts

[edit]

Xen can be shipped in a dedicated virtualization platform, such as XCP-ng or XenServer (formerly Citrix Hypervisor, and before that Citrix XenServer, and before that XenSource's XenEnterprise).

Alternatively, Xen is distributed as an optional configuration of many standard operating systems. Xen is available for and distributed with:

Guests

[edit]

Guest systems can run fully virtualized (which requires hardware support), paravirtualized (which requires a modified guest operating system), or fully virtualized with paravirtualized drivers (PVHVM[74]).[75] Most operating systems which can run on PCs can run as a Xen HVM guest. The following systems can operate as paravirtualized Xen guests:

Xen version 3.0 introduced the capability to run Microsoft Windows as a guest operating system unmodified if the host machine's processor supports hardware virtualization provided by Intel VT-x (formerly codenamed Vanderpool) or AMD-V (formerly codenamed Pacifica). During the development of Xen 1.x, Microsoft Research, along with the University of Cambridge Operating System group, developed a port of Windows XP to Xen — made possible by Microsoft's Academic Licensing Program. The terms of this license do not allow the publication of this port, although documentation of the experience appears in the original Xen SOSP paper.[79] James Harper and the Xen open-source community have started developing free software paravirtualization drivers for Windows. These provide front-end drivers for the Xen block and network devices and allow much higher disk and network performance for Windows systems running in HVM mode. Without these drivers all disk and network traffic has to be processed through QEMU-DM.[80] Subsequently, Citrix has released under a BSD license (and continues to maintain) PV drivers for Windows.[81]

Management

[edit]

Third-party developers have built a number of tools (known as Xen Management Consoles) to facilitate the common tasks of administering a Xen host, such as configuring, starting, monitoring and stopping of Xen guests. Examples include:

  • The OpenNebula cloud management toolkit
  • On openSUSE YaST and virt-man offer graphical VM management
  • OpenStack natively supports Xen as a Hypervisor/Compute target
  • Apache CloudStack also supports Xen as a Hypervisor
  • Novell's PlateSpin Orchestrate also manages Xen virtual machines for Xen shipping in SUSE Linux Enterprise Server.
  • Xen Orchestra for both XCP-ng and Citrix Hypervisor platforms

Commercial versions

[edit]

The Xen hypervisor is covered by the GNU General Public Licence, so all of these versions contain a core of free software with source code. However, many of them contain proprietary additions.

See also

[edit]

References

[edit]

Further reading

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Xen

View on Grokipedia
from Grokipedia
Xen is an open-source type-1 (bare-metal) that enables the secure execution of multiple virtual machines, each running an independent operating system, on a single physical host by directly managing hardware resources without relying on a host operating system. Developed under the Xen Project, a global open-source initiative hosted by the , Xen supports both paravirtualization (PV), where guest operating systems are modified for optimal performance, and full hardware-assisted (HVM), allowing unmodified guest OSes to run. Licensed under the GNU General Public License version 2 (GPLv2), it emphasizes security, efficiency, and scalability across diverse architectures including x86 and . The origins of Xen trace back to the late 1990s at the Computer Laboratory, where researchers sought to advance technology for x86 systems. The project released its first public version on October 3, 2003, supporting 2.4.22 as a guest OS and marking a milestone in open-source hypervisors. In 2005, Xen 3.0 introduced support for VT-x , broadening its applicability. The formation of XenSource in 2005 led to commercial adoption, culminating in Citrix's $500 million acquisition of the company in 2007, which further propelled its development. By 2013, the Xen Project joined the , fostering a merit-based community governance model involving contributors from companies like , , Citrix, and . Key features of Xen include its ring-0 hypervisor design for minimal privileged code execution, enhancing security through isolation of virtual machines (domains), with Domain 0 serving as the privileged control domain for management tasks. It supports live migration of virtual machines between hosts without downtime, non-disruptive patching since Xen 4.8 in 2016, and advanced I/O virtualization via paravirtualized drivers for improved performance. The latest stable release, Xen 4.20, was announced in March 2025, introducing enhanced security features and performance optimizations. Xen also integrates with unikernel technologies like Mirage OS and Unikraft for lightweight, secure applications, and provides enterprise tools such as XAPI for cluster management. Its scheduler options, including credit-based and real-time variants, cater to varied workloads from cloud computing to real-time embedded systems. Xen powers critical infrastructure worldwide, notably serving as the foundational for early versions of ' EC2 launched in 2006, handling millions of virtual instances daily. It underpins commercial platforms like Citrix Hypervisor (formerly XenServer), the community-driven distribution, and VM Server, supporting enterprise data centers and private clouds. In embedded and automotive sectors, Xen enables mixed-criticality systems, with support since 2008 and efforts toward safety certifications for compliance ongoing since 2018, achieving a major milestone in late 2024. Security-focused deployments include for compartmentalized computing and Bitdefender's virtual machine introspection tools since 2017, while its influence extends to over 10 million daily users across servers, desktops, and IoT devices.

History

Origins and Development

The Xen hypervisor originated from the XenoServers project, initiated in 1999 at the Computer Laboratory under the leadership of Dr. Ian Pratt and a team of researchers. This effort aimed to create a global-scale, public computing infrastructure capable of safely hosting untrusted programs and services across distributed nodes, addressing the need for accountable execution in wide-area networks. By 2003, the project evolved into the development of Xen as a research initiative focused on paravirtualization, a technique that modifies guest operating systems to cooperate with the hypervisor for improved efficiency. The primary motivation was to overcome the performance overheads of full virtualization—such as those from binary translation and trap handling in earlier systems like VMware—making it suitable for performance-critical workloads where unmodified binaries proved inefficient. Ian Pratt, along with collaborators including Keir Fraser, Steven Hand, and Christian Limpach, released the first version of Xen that year, demonstrating its ability to host multiple commodity operating systems on x86 hardware with near-native performance. A significant milestone occurred in 2007 when acquired XenSource, the company founded by Pratt and other Cambridge researchers to commercialize Xen, for approximately $500 million. This deal accelerated Xen's adoption in enterprise environments while maintaining its open-source roots. In 2013, the Xen Project was established as a collaborative project under the to provide neutral governance, fostering broader community involvement and ensuring long-term sustainability. Key industry contributions have since solidified Xen's evolution, including hardware-specific enhancements from and to leverage their AMD-V and Intel VT-x virtualization extensions for better isolation and efficiency. Amazon Web Services (AWS) has also played a pivotal role, powering its Elastic Compute Cloud (EC2) service with Xen and contributing upstream improvements for scalability in cloud deployments.

Release History

The Xen hypervisor's first public release, version 1.0, occurred in 2003 and introduced basic paravirtualization capabilities primarily for guest operating systems, enabling efficient resource sharing among virtual machines on x86 hardware. In December 2005, Xen 3.0 was released, marking a significant advancement with the addition of hardware-assisted (HVM) support, which allowed unmodified guest operating systems to run without paravirtualization modifications by leveraging VT-x and AMD-V extensions. The project transitioned to the Xen 4.x series with the release of version 4.0 in April 2010, initiating a pattern of iterative improvements focused on stability, , and broader hardware compatibility under the governance of the Xen Project, hosted by the since 2013. Subsequent releases in the 4.x series have followed an approximately annual cadence for major versions. For instance, Xen 4.19, released on July 31, 2024, delivered performance boosts through optimizations in and I/O handling, alongside enhancements. The series deprecated the older xm toolstack in favor of the xl toolstack starting with Xen 4.1 in 2011, with xm fully removed by Xen 4.5 in 2015 to streamline management interfaces. As of November 2025, the latest stable release is Xen 4.20 from March 5, 2025, which includes enhanced patches such as expanded compliance for code quality and ARM64 improvements like support for Armv8-R profiles and last-level cache coloring.

Architecture

Core Software Architecture

Xen operates as a type-1 , executing directly on the physical hardware in the most privileged mode, known as Ring 0 on x86 architectures, where it manages core resources such as CPU scheduling, memory allocation, and interrupt handling without an underlying host operating system. This bare-metal design ensures high performance and security by minimizing the , with the itself comprising a small, focused focused on essentials. At the heart of Xen's architecture is the , where virtual machines are termed domains. The initial domain, Dom0, is automatically created during boot and serves as the privileged control domain, possessing exclusive access to physical hardware for device management, including I/O operations and to other domains. Unprivileged domains, referred to as DomU, run guest operating systems and can be either paravirtualized (PV) guests, which are aware of the hypervisor and use modified interfaces for direct interaction, or hardware virtualized (HVM) guests, which leverage hardware extensions for compatibility with unmodified operating systems. Dom0 typically runs a full-featured operating system like , which hosts essential drivers and management tools, while DomU domains operate in a sandboxed environment with restricted privileges. Xen's design adopts a microkernel-like approach, intentionally limiting the to a minimal —around 90,000 lines of code for implementations as of 2025—to enhance stability and reduce attack surfaces, with no device drivers or complex services embedded within it. Instead, higher-level functionality such as storage, networking, and user-space management is delegated to Dom0 or external toolstacks like xl or libvirt, allowing for modular updates without compromising the hypervisor's integrity. Efficient inter-domain communication is facilitated by event channels and grant tables, core primitives that enable secure and performant resource sharing. Event channels act as lightweight virtual interrupts, allowing domains to signal each other asynchronously; they are created and managed via hypercalls like HYPERVISOR_event_channel_op, supporting thousands of channels per domain via the FIFO ABI since Xen 4.4, with limits exceeding 100,000 for . Grant tables provide a mechanism for controlled sharing, using grant references to permit temporary access to pages without full emulation or copying, as seen in operations like gnttab_grant_foreign_access for block devices or gnttab_grant_foreign_transfer for network transfers, ensuring isolation while avoiding overhead. These mechanisms underpin paravirtualized I/O protocols, where frontend drivers in DomU connect to backends in Dom0 via rings notified through event channels.

Virtualization Techniques

Xen employs several virtualization techniques to enable the execution of guest operating systems on virtualized hardware, primarily through paravirtualization and hardware-assisted methods. These approaches allow Xen to balance performance, compatibility, and security by adapting guest interactions with the and underlying hardware. The core techniques include paravirtualization (PV), hardware virtual machine (HVM), and the hybrid PVH mode, each tailored to different guest requirements. In paravirtualization (PV), guest operating systems are modified to recognize their virtualized environment and communicate directly with the . These modifications involve minimal changes to the guest kernel, such as replacing hardware-specific drivers with paravirtualized interfaces that issue hypercalls—software traps analogous to system calls—for resource access. Hypercalls handle critical operations like page-table updates, I/O requests, and CPU scheduling, enabling the to multiplex resources efficiently among domains without emulating hardware. For I/O, guests enqueue requests using asynchronous ring buffers shared with the hypervisor, which forwards them to backend drivers in the privileged Domain 0 (Dom0), allowing Xen to reorder operations for scheduling or priority without ambiguity. CPU scheduling in PV guests relies on hypervisor-managed policies, such as the Borrowed Virtual Time (BVT) algorithm, invoked via hypercalls to yield control or request time slices. This technique, introduced in early Xen versions, requires access to the guest OS but provides strong isolation by running guests in ring 1 privilege level while the hypervisor operates in ring 0 (on x86). Hardware-assisted virtualization (HVM) supports unmodified guest operating systems by leveraging CPU extensions like VT-x or AMD-V to handle sensitive instructions and transitions transparently. In HVM mode, the guest runs as if on bare metal, with the trapping and emulating privileged operations that cannot be directly executed. Device emulation, including , IDE controllers, VGA, USB, and network interfaces, is provided by a device model (typically ) running in Dom0, which mediates I/O between the guest and physical hardware. in HVM primarily employs hardware-assisted paging with extensions like EPT or NPT, with shadow page tables used as a fallback. handling in HVM emulates controllers like APICs and IOAPICs, with upstream IRQ delivery routed through the to the guest via emulated mechanisms, though paravirtualized drivers can enhance this by using event channels for more direct notification. HVM thus enables broad compatibility, such as running proprietary OSes like Windows, at the cost of additional emulation overhead. PVH represents a hybrid virtualization mode that combines the efficiency of paravirtualization with the compatibility of HVM, targeting 64-bit guests booted in a lightweight HVM container without full device emulation. Introduced in Xen 4.4 for DomU guests and extended to Dom0 in Xen 4.5, PVH uses extensions (VT-x or AMD-V) for core operations like paging and CPU context switches, while incorporating PV-style hypercalls for , mapping, and device access to reduce the emulation burden. Guests via a PV mechanism, such as ELF notes for the kernel, but execute at native privilege level 0 within the HVM context, eliminating the need for ring compression and minimizing guest modifications. For security, PVH enhances isolation by avoiding emulated devices and relying on hardware MMU virtualization, which reduces the compared to traditional PV modes that expose more interfaces. Specific hypercalls in PVH include XENMEM_memory_map for retrieving the e820 , PHYSDEVOP_* for IRQ and device setup, HVMOP_set_param for configuration, and VCPUOP_* for processor operations, enabling direct communication without a separate device model. This mode supports upstream IRQ handling through event channels, similar to PV, and uses hardware-assisted paging to supplant shadow tables where possible.

ARM-Specific Architecture

On ARM architectures, Xen runs in EL2 (Exception Level 2), the mode, managing resources via stage-2 memory translations for guest isolation, analogous to x86's EPT/NPT. ARM guests operate in EL1 (kernel) or EL0 (user), with extensions (ARMv7 VE or ARMv8) enabling HVM-like support without software shadow paging. Paravirtualization on ARM uses hypercalls similar to x86 but leverages ARM's GIC (Generic Interrupt Controller) for event channels and SMMU for . This design ensures efficiency in embedded and server environments, with no ring compression needed due to ARM's flat privilege levels.

Features

Security and Isolation

Xen employs the Xen Security Modules (XSM) framework, which provides a flexible (MAC) system to enforce fine-grained policies across domains. The primary implementation, XSM-FLASK, integrates the FLASK —developed by the NSA as an analog to SELinux—allowing administrators to define policies that control domain creation, access, and inter-domain communications using SELinux-compatible tools and syntax. This enables robust isolation by restricting unauthorized interactions, such as preventing unprivileged domains from accessing sensitive resources or other guests' memory. At the core of Xen's isolation model is the prohibition of direct memory access between domains, ensuring that guests cannot arbitrarily read or write to each other's address spaces or the hypervisor's. Instead, controlled memory sharing is facilitated through grant tables, a mechanism where a domain explicitly grants temporary access to specific pages via hypercalls, with the hypervisor mediating all transfers to maintain integrity and confidentiality. This design mitigates time-of-check-to-time-of-use (TOCTOU) vulnerabilities that could arise in shared memory scenarios, as any modifications trap into the hypervisor for validation, preventing race conditions during access grants. By leveraging shadow page tables and event channels for notifications, Xen further enforces strict separation, reducing the attack surface even in paravirtualized environments. As of 2025, the Xen Project is actively developing support for technologies like SEV-SNP and TDX, with integration expected in future releases. To address historical vulnerabilities like the 2015 flaw (CVE-2015-3456), which exploited QEMU's controller emulation for guest-to-host escapes, Xen utilizes its split device model to isolate device emulation in dedicated driver domains rather than the control domain (Dom0). This architecture confines potential exploits to less-privileged domains, limiting and allowing independent restarts without affecting the . Complementary measures include verified mechanisms, which cryptographically validate and guest images during startup using tools like shim and GRUB with Secure Boot support, ensuring only trusted code executes and mitigating supply-chain attacks. These combined strategies have hardened Xen against escape vectors, with ongoing security advisories addressing emergent threats through policy enforcement and hardware isolation.

Performance Optimizations

Xen employs the Credit2 scheduler as its default mechanism for dynamic CPU allocation across virtual machines, known as domains, enabling efficient resource sharing and overcommitment where more virtual CPUs can be allocated than physical ones available. This scheduler prioritizes fairness, responsiveness, and scalability, particularly for mixed workloads, by assigning credits based on domain weights and adjusting allocations in real-time to prevent while maximizing throughput. Live migration in Xen, branded as XenMotion in distributions like XenServer, facilitates zero- movement of running virtual machines between physical hosts, preserving workload continuity during maintenance or load balancing. This process involves iteratively transferring memory pages and CPU state, with convergence ensured through techniques like pre-copy and post-copy to minimize to under a second. Storage live migration extends this capability by relocating virtual disk images alongside the VM when shared storage is unavailable, achieving seamless transitions without interrupting I/O operations. For I/O optimization, Xen leverages Virtio drivers in paravirtualized (PV) guests to provide semi-virtualized interfaces that reduce overhead compared to fully emulated devices, yielding up to 90% of native in disk and network operations. In PV mode, these drivers enable direct communication between guest kernels and backend services in the control domain, bypassing slower emulation paths. Additionally, SR-IOV passthrough allows direct assignment of physical network functions to VMs, bypassing the entirely for near-native throughput—often exceeding 95% of bare-metal speeds—while supporting scalability for high-bandwidth applications like cloud networking. Recent advancements in Xen versions 4.19 and 4.20, released in 2024 and 2025 respectively, have enhanced architecture support through improved hardware compatibility and extensions.

Deployment

Supported Hosts

Xen primarily supports hardware platforms from and processors as the host environment for running the hypervisor. These systems require extensions, such as VT-x or AMD-V (SVM), to enable full HVM (Hardware Virtual Machine) guests, while paravirtualized (PV) guests can run without them. Additionally, for advanced features like device passthrough, an IOMMU such as VT-d or AMD-Vi is recommended and often required in production setups to ensure secure memory isolation. Support for architectures was introduced in Xen version 4.3, enabling deployment on compatible server hardware like those from or processors. hosts also necessitate virtualization extensions (ARMv8 VE) for HVM operation, with up to 128 physical CPUs supported in recent releases. Experimental builds for architectures became available starting with Xen 4.20 in early 2025, targeting emerging hardware but remaining in a tech preview status without full production stability. The primary operating system for the control domain (Dom0), which manages the , is , with support integrated into the mainline kernel since version 3.0. Compatible distributions include , , (and its successor ), , and Gentoo, all requiring a Xen-enabled kernel configured with the necessary tools like xl for domain management. has offered Dom0 support since version 11.0, with enhancements for booting in 14.0 and later. For optimal performance, Dom0 should use a minimal kernel configuration to reduce overhead, incorporating Xen-specific modules and, in production environments, enabling IOMMU for secure device assignment, including improved GPU passthrough capabilities tailored for AI workloads on x86 and platforms.

Supported Guests

Xen supports three primary virtualization modes for guest operating systems: paravirtualized (PV), hardware virtual machine (HVM), and paravirtualized hardware (PVH). Each mode offers varying levels of compatibility and performance, with PV requiring guest kernel modifications for optimal integration, HVM allowing unmodified guests via , and PVH combining with paravirtualization for enhanced security and efficiency. PV guests necessitate modifications to the guest operating system's kernel to enable direct communication with the Xen , bypassing the need for extensions. Supported operating systems include most distributions using kernels version 2.6.24 or later with pvops support, , and historical versions of Solaris that include PV drivers. also runs as a PV guest with appropriate kernel ports. This mode is suitable for legacy environments or workloads without hardware virtualization, though it is increasingly deprecated in favor of PVH. HVM guests operate without kernel changes, leveraging Intel VT-x or AMD-V extensions and for device emulation to support unmodified operating systems. Windows versions up to 11 and Server 2025 run as HVM guests, with performance enhanced by optional PV drivers provided by Citrix for storage, networking, and . Various BSD variants, including and , function via HVM emulation, while HVM support is available. Linux distributions such as RHEL 8/9, 20.04/22.04/24.04, and 11/12 are fully supported in HVM mode, often requiring XenServer VM Tools for optimal integration. PVH guests utilize for boot and control while employing paravirtualized interfaces for I/O, eliminating the need for emulation and reducing the compared to HVM. This mode is primarily supported by modern kernels version 4.11 and later, providing improved for 64-bit environments. Windows support in PVH is not native but can be achieved through Citrix tools that install PV drivers post-boot in compatible HVM setups transitioning to PVH-like behavior. Key limitations include the absence of native Android support across all modes, relying instead on emulation layers that are not officially endorsed.

Applications

Common Uses

Xen is widely deployed in environments to provide scalable (IaaS). Early instances of ' Elastic Compute Cloud (EC2) relied on the Xen hypervisor for virtualization, enabling efficient resource sharing and high availability until the transition to the Nitro system in 2017. Additionally, Xen integrates seamlessly with , allowing operators to manage virtual machines across diverse hardware while supporting para-virtualized and hardware-assisted modes for robust IaaS deployments. In enterprise settings, Xen facilitates server consolidation by allowing multiple virtual machines to run on a single physical host, reducing hardware costs and improving energy efficiency. It powers virtual desktop infrastructure (VDI) solutions, particularly through Citrix Virtual Apps and Desktops, where XenServer provides optimized isolation and for delivering secure, remote desktops to end-users. This enables organizations to centralize management while supporting demanding workloads like application delivery. Xen supports security-focused applications, including Qubes OS, which uses the hypervisor for compartmentalized desktop computing to isolate tasks and enhance privacy and security. It also enables advanced threat detection through tools like Bitdefender's Hypervisor-based Memory Introspection (HVMI), which leverages Xen's virtual machine introspection APIs to monitor guest memory for malware without agents inside VMs. For edge computing in IoT and automotive scenarios, Xen's paravirtualization mode offers low-overhead virtualization, making it suitable for resource-constrained devices by minimizing performance penalties and enabling isolated execution of multiple services on gateways or embedded systems. In automotive applications, Xen facilitates mixed-criticality systems for software-defined vehicles (SDV), with ongoing efforts toward ISO 26262 safety certification and real-time support for safety-critical workloads, as demonstrated by deployments like Honda's SDV development in 2025. Its lightweight architecture supports data processing near the source, reducing latency in distributed IoT networks. Xen is employed in (HPC) for scientific simulations, where its low virtualization overhead—often under 2% for compute-intensive tasks—allows near-native performance in virtualized clusters. Techniques like sidecore allocation and self-virtualized I/O further optimize multi-core , making it viable for fault-tolerant environments running MPI-based applications. Emerging 2025 trends highlight Xen's role in AI/ML workloads through GPU passthrough, which assigns dedicated graphics processing units to virtual machines for accelerated training and inference with minimal latency overhead. Xen's strong isolation features enable secure processing of sensitive data in multi-tenant setups.

Management and Tooling

The primary toolstack for managing Xen environments is the xl command-line interface, which has been the default since Xen 4.5 and is built on the libxl C library for lightweight operations such as domain creation, , and real-time monitoring. xl supports dynamic configuration changes during runtime, preserving modifications across domain lifecycle events like suspend and resume, which were further enhanced in Xen 4.20 with dedicated subcommands for these operations. Alternative toolstacks provide flexibility for specific deployments; XAPI serves as the management interface for XenServer (now part of Citrix Hypervisor), handling VM lifecycle, networking, and storage across pooled hosts in enterprise settings. For broader ecosystem compatibility, Xen integrates with libvirt through its libxl driver, enabling unified management of Xen domains alongside other hypervisors like KVM via APIs for domain provisioning and control. Monitoring in Xen environments leverages integrations with open-source tools like for metrics collection via exporters such as xen-exporter, which exposes host and guest performance data including CPU and memory utilization. This data can be visualized in dashboards tailored for Xen, providing dashboards for critical metrics across or XenServer pools. For debugging, xentrace captures trace buffer events from the in binary format, allowing analysis of low-level operations like context switches and interrupts to diagnose performance issues. In 2025 developments, discussions at Xen Summit highlighted proposals for a modular toolstack to improve scalability, particularly for platforms like Altra, building on xl's existing support while addressing efficiency needs. Xen 4.20, released in March 2025, reduced dependencies in the xenstore library to streamline tooling and added command-line options for time source selection, enhancing administrative precision.

Availability

Open-Source Distributions

The Xen Project maintains official open-source releases of the , providing repositories and pre-built binaries downloadable from xenproject.org. These releases, such as Xen 4.20 issued in March 2025, support a range of architectures and include enhancements for security and performance. The project hosts its primary mirror on , enabling developers to clone, build, and contribute via standard practices. Xen is integrated into major Linux distributions as a native component, allowing straightforward installation through their package managers. For instance, Fedora includes Xen packages that can be installed via DNF, turning a standard installation into a Xen host with minimal configuration. Similarly, SUSE Linux Enterprise Server provides comprehensive Xen support, with documentation for setting up hosts and managing virtual machines directly from YaST or Zypper. Community initiatives extend Xen's usability through dedicated open-source projects. , a of the original XenServer, delivers a fully open-source platform with integrated management tools, emphasizing unrestricted access to features like and high-availability clustering. This project maintains compatibility with upstream Xen releases while adding community-driven enhancements for enterprise-like deployments. , an open-source management platform, supports importing virtual machines from Xen environments using tools like virt-v2v, facilitating migrations to KVM-based setups. Installation of Xen on Linux systems typically involves upstream kernel modules for paravirtualization support or Dynamic Kernel Module Support (DKMS) to automatically rebuild modules during kernel updates, ensuring compatibility across distro versions. Packages are readily available in repositories for distributions like , , , and openSUSE, often requiring only commands like apt install xen-system-amd64 or dnf install xen. As of 2025, the Xen Project features an active port, with Xen 4.20 providing initial enhancements for support, including improvements in device tree mapping and initialization, alongside ongoing development for advanced features like extensions. Contributions from the Xen community to drivers continue to improve guest performance and integration, including updates to paravirtualized block and network interfaces in recent kernel releases.

Commercial Versions

Citrix Hypervisor, formerly known as XenServer, is a leading commercial implementation of the Xen hypervisor, providing enterprise-grade with subscription-based support and integrated management tools. As of October 2025, the latest release is XenServer 8.4, which includes support for guest operating systems, including virtual Trusted Platform Modules (vTPM) for enhanced security compliance. This version also features integrated Citrix Provisioning Services (PVS) acceleration for efficient image deployment and monitoring capabilities via SNMP and , enabling robust enterprise monitoring and automation. Oracle VM Server for x86 remains a commercial distribution based on the , offering enterprise management through and compatibility with Oracle's ecosystem, including database appliances. As of 2025, is in sustaining support (since October 2020), providing indefinite access to existing releases but no new security patches or updates for emerging vulnerabilities. It supports a range of guest operating systems, including Windows and , with tools for and tailored to Oracle's standards. Other vendors, such as , have developed customized commercial platforms incorporating Xen for and deployments, though recent iterations of Huawei's FusionSphere emphasize KVM-based while maintaining compatibility with Xen hypervisors. These implementations often include specialized features for carrier-grade reliability, such as extended I/O and recovery options optimized for telco networks. In response to the 2024 Broadcom acquisition of and subsequent licensing changes, Citrix has enhanced migration tools for transitioning from environments to Xen-based platforms. The Conversion Manager appliance facilitates batch conversion of VMs to XenServer, preserving networking and storage configurations for seamless deployment. This tool supports parallel migrations, making it a key resource for organizations seeking cost-effective alternatives in 2025.

References

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