You can't really define the difference because there are so many different types of "classical" playing and so many different types of "folk" playing.

There's some crossover in the early music world, and jazz violin is in a category of its own, as is the style in which Indian music is played on the violin.

BTW orchestral musicians talk about violins as fiddles, but that's definitely slang and derogatory, though never taken seriously as an insult.

Anahata