Hardanger fiddle: Violin ❌, Fiddle ✅
The "big fiddle" (i.e., cello) from a wee fiddle and big fiddle ensemble: Violin ❌, Fiddle ✅
Cigar-box fiddle: Violin ❌, Fiddle ✅
Guarnieri violin: try handing it to a fiddler - they can probably make it work. Violin ✅, Fiddle ✅

My (not-entirely-serious) opinion, setting aside the cultural/stylistic differences and considering the physical instruments themselves, is that all violins are (or at least can be) fiddles but not all fiddles are violins.