I am asked this question often.
The answer is none.
Though classical violinists often have very expensive instruments and fiddlers might have more humble instruments, there is no difference between a violin and a fiddle.
Now there is much discussion here as I discovered one night talking with a Bluegrass musician friend of mine. A fiddle might have a flatter curvature to the bridge and possibly use a different type of string.
I recently went to the Bill Monroe Bluegrass Festival at his old homestead in Kentucky and a vendor was selling hundreds of fiddles. He told me that people in those parts never payed more than $300 for a fiddle.
Of course classical violinists have to play in higher registers and need a violin that projects in a concert hall. Whereas fiddlers play mostly in the lower positions.
Whether you have a violin or a fiddle is determined by what type of music you play on your instrument.