The output for the violin and piano repertoire of Wolfgang Amadeus Mozart is very extensive. The whole set, from the first child-pieces till the last one reaches up to 36 sonatas. With the first pieces for this repertoire being KV.10-15, they show the beginning composition style of the young genius.
Most of the early sonata (sets) are written during the travels that he made as a child-prodigy. The first set was written during a trip that he and he father made in Paris during January 1764. Mozart was not only known for his virtuosity on the piano but also as a very good violinist, it is very likely that he played his own violin concerto's as solist. , which he completed over a course of several months at age 19. It is known that he had a special appreciation for the viola, which can be seen in the Sinfonia Concertante (kv.364) and the 'Kegelstatt'-Trio for Piano, Clarinet (or Violin) and Viola (Kv.498)
Not only are these young violin sonatas (including as well the set of Kv301-306) written for the violin but for flute as well. Mozart did as well compose them for Harpsichord, which was a more common instrument at that time than the forte-piano. In November 1781 the Kv.296 Sonata in C major was published by Artaria with the title: ("six sonatas pour le Clavecin ou Pianoforte, avec l'accompaniment d'un violin").
We should as well take into consideration that, especially the young works, were written not for professionals but for amateurs. It was not up until Kv.306 that Mozart wrote in a more grandeur style that demands more from the performer than anything he wrote before for both instruments. Certain sources even suggest that Mozart would not mind to play them without the violin part since most households had a Clavichord, Harpsichord or sometimes even a forte-piano:
"The sonatas for violin and pianoforte (or harpsichord at first) are more properly reviewed with those for keyboard alone than with the chamber music. The eighteenth-century convention was that such works should be essentially keyboard music. Sonatas were written for clavier. The violin could join in if it was available (or in England the alternative 'German flute'), but the composer was not going to have the popularity of his domestic music threatened by the comparative rarity of players on that instrument. Every household had harpsichord or clavichord, displayed some time during the second half of the century by the rapidly spreading pianoforte, and sonatas were so devised that they could be played just as well without a second instrument, though their effect was heightened by its participation" ("Mozart, The Master Musicians", Eric Blom P.253)
The way that Mozart uses the violin in the early sonatas can often be seen as a mere accompaniment. Often being made in thirds, which serves more as a color-function, filling with chord-tones often in the middle-register below the right hand of the piano, making the piano the melodic line, this changes in the later sonatas and one can see especially in the sonata Kv.454 that the instruments become equal parts.
During his travels to Mannheim in 1777, Mozart had the opportunity to see the harpsichord-violin 'duets' of Joseph Schuster (a Kapellmeister of the Elector of Saxony) who composed in a rather different style. After this, Mozart started to change his composition style, giving a more important role to the violin, which resulted in the sonatas Kv 301-306, which are more mature than his earlier sonatas that he wrote as a child.
Mozart's style in the piano and violin sonatas rapidly changed after those six sonatas which resulted in another set including the major sonata in B-flat major Kv.378, G major/minor Kv.379 and others. These sonatas have a much grander layout and some movements hint towards concerto style, although the Kv.306 already shows a concertante style and has a double concerto for violin-piano and orchestra that he left unfinished which is based upon the Kv.306.
The latest sonatas sometimes have the model of a Trio and it is known as well that earlier sonatas were sometimes played by a cello.