Xavier, Duke of Parma and Piacenza, known in France before 1974 as Prince Xavier de Bourbon-Parme, known in Spain as Francisco Javier de Borbón-Parma y de Braganza or simply as Don Javier (25 May 1889 – 7 May 1977), was head of the ducal House of Bourbon-Parma. He is best known as dynastic leader of Carlism and the Carlist pretender to the throne of Spain, since 1936 as a regent-claimant and since 1952 as a claimant, appearing under the name Javier I. Since 1974, he was pretender to the defunct throne of Parma. He is also recognized as involved in the so-called Sixtus Affair of 1916–1917 and in the so-called Halifax-Chevalier talks of 1940.

Xavier was born into the House of Bourbon-Parma, an Italian cadet branch of the Spanish Bourbons, the royal family of Spain, who in turn had diverged from the French House of Bourbon in the 18th century. Xavier was a patrilineal descendant of Louis XIV and to the king of Spain Felipe V. Among his great-great-grandparents, Ludovico I was the king of Etruria, Vittorio Emanuele I was the king of Sardinia and the duke of Savoy, Charles X was the king of France, Francesco I was the king of Two Sicilies, Pedro III was the king of Portugal, Maria I was the queen of Portugal and Brazil, and Carlos IV was the king of Spain; among his great-grandparents, Carlo II was the duke of Parma and João VI was the king of Portugal; among his grandparents, Carlo III was the duke of Parma and Miguel I was the king of Portugal. Xavier's father, Robert I (1848–1907) was the last ruling duke of Parma, and Xavier's mother, Infanta Maria Antónia of Portugal (1862–1959), was exile-born daughter of the 1834-deposed king of Portugal.

Many of Xavier's uncles and aunts came from European royal or ducal families, though the only one actually ruling was his mother's sister, Infanta Maria Ana of Portugal, Grand Duchess of Luxembourg. The other three were claiming the throne: his mother's brother, the Portuguese Miguelist pretender Dom Miguel, Duke of Braganza, his father's sister, the Carlist queen of Spain Margarita de Borbón-Parma and another sister of his mother, also the Carlist queen of Spain, Infanta Maria das Neves of Portugal. One uncle, archduke Karl Ludwig, was official heir to the throne of Austro-Hungary. Of Xavier's cousins the only two who actually ruled were Elisabeth, the queen consort of Belgium and Charlotte, the Grand Duchess of Luxembourg. Xavier's step-cousin, archduke Franz Ferdinand, was official heir to the Austro-Hungarian throne. Two cousins were legitimist pretenders; along the paternal line Don Jaime, the Carlist claimant to the Spanish throne, and along the maternal line Dom Duarte Nuño, the Miguelist claimant to the Portuguese throne.

Some of Xavier's siblings have married into the ruling European houses and few have actually ruled: these were the cases of his younger sister Zita, who in 1911 married into the imperial Habsburg family to become the empress of Austria and the queen of Hungary in 1916–1918, and this of his younger brother Felix, who in 1919 married into the ducal Nassau family and was the duke-consort of Luxembourg in 1919–1970. Some of Xavier's siblings were closely related to actual rulers: these were the cases of his younger brother René, who in 1921 married into the royal Danish family, this of his younger brother Louis, who in 1939 married into the royal Italian family, and this of his older half-sister Maria Luisa, who in 1893 married into the royal Bulgarian family. Some of Xavier's siblings married into ducal or otherwise distinguished highly aristocratic houses. Six mentally handicapped older half-siblings have never married and three of Xavier's sisters became Benedictine nuns.

Though deposed as Duke of Parma in 1859, Xavier's father kept claiming the title. He retained massive wealth, comprising estates in Italy and Austria; moreover, in the late 19th century the Bourbon-Parma inherited the magnificent Chambord castle. The family, consisting of Robert, his second wife and some 20 children from both marriages lived in two homes, in Pianore and in Schwarzau. They used to spend half a year in each location, shuttling in a special train and taking even children's horses with them. Xavier's childhood was full of serenity, luxury and cheerfulness, though relations with half-siblings from the first marriage were not equally cordial. The Bourbon-Parma were deeply Roman Catholic and essentially French in culture and understanding; another language spoken was German. In his childhood Xavier picked up also Italian – spoken with the Pianore locals, English – spoken with various visitors, Portuguese and Spanish – used in certain relations, and Latin – used in church. The family were frequently visited by guests from the world of aristocracy, books and universities.

In 1899 Xavier followed in the footsteps of his older brother Sixte and entered Stella Matutina, a prestigious Jesuit establishment in the Austrian Feldkirch. Though catering to Catholic aristocracy from all over Europe, the school offered Spartan conditions; when later enquired how he survived the Nazi concentration camp, prince Xavier joked: "I attended the Stella. It's not easy to kill us". The school ensured a model of humble religiosity, the staff ensured high teaching standards, and the mix of boys from different countries ensured a spirit of international comradeship. Xavier graduated in the mid-1900s; in 1906 moved to Paris, still trailing his older brother and commencing university studies. Unlike Sixte, who studied law, he pursued two different paths: political-economic sciences and agronomy. He completed both, graduating as engineer in agronomy and doctor in politics/economy. The year or years of him completing the curriculum are not clear; one source points to 1914. He has never commenced a professional career.

In 1910 the wealth of the late Robert Bourbon-Parma was divided among the family. Children from the first marriage, and especially Élie, custodian of his handicapped siblings, were allocated most of the real estate; Robert's second wife and children from the second marriage were earmarked hefty financial compensation, usufruct rights and minor properties. Already on his own, Xavier was based in Paris but cruised across Europe. One reason was family business, often with political background; e.g. in 1911 Xavier travelled to Austria to attend the wedding of his sister with archduke Karl Habsburg, heir to the Austro-Hungarian throne; in 1912 he travelled via Spain to Portugal, accompanying his aunt during a Portuguese legitimist plot. Another reason was following his personal interest. Xavier seemed heavily influenced by Sixte, who developed a knack for geographical exploration. In 1909 both brothers travelled to the Balkans; in 1912 they roamed across Egypt, Palestine and the Near East. In 1914 they intended to travel to Persia, India and possibly the Himalayas.

News of the Sarajevo assassination reached Xavier and Sixte in Austria, en route to Asia. Enraged by murder of their step-cousin, both brothers intended to enlist in the Austrian army to pursue revenge. Things changed when France declared war on Vienna. Though some of the Bourbon-Parma siblings – Zita, René, Felix and Élie – sided with Austria-Hungary, with males joining the imperial troops, Xavier and Sixte felt themselves thorough Frenchmen. They openly made plans to enlist in the French army, which might have evoked their detention. It took personal appeals from Zita before the Emperor took steps which prevented their incarceration, and allowed them to leave Austria for a neutral country. When back in France Xavier and Sixte indeed volunteered, only to find that French law banned members of foreign dynasties from serving. Determined to fight, they contacted their cousin, Queen Elisabeth of Belgium, who saw to it that both were allowed to serve in the Belgian military. Due to a car accident involving Sixte, however, the brothers joined the ranks of the Belgian army no sooner than late November 1914. Xavier was initially accepted as a private in medical services and was seconded to the 7th artillery regiment. Exact details of his service are not clear; what was left of the pre-war Belgian army served on a relatively calm sector of the frontline in Flanders and France, next to the English Channel. At an unspecified time Xavier was released from the line and assigned to an officer training course, organized by the Belgian general staff, which he completed. By mid-1916 he was sub-lieutenant, later promoted to captain.