To clerifiy, are you looking for a "violin" meaning a solo classical instrument, or a "fiddle" meaning a solo instrument used in country music.
While technically the same instrument, the bowing techniques, tone and "licks" are much different.
There are several good orchetral sample libraries (Miroslav, etc) that can provide very realistic sounding solo violin. There is a country instrument sample library (can't remember the name right now) that came in a few formats (Akai, etc) this country sample library is also available as a sound expansion board for the Roland JV/XP products.
However, keyboard playing technique is so very important to get a realistic performance (anytime you try create accoustic instrument performances using a keyboard the ability to play "realistic licks" is as important as the actual sound).
"A violin. Played by a professional. There is no substitute." is the correct response, however I have found that violin is a very hard instrument to properly record. The wrong room or the wrong mic can make a beautiful violin sound like a harsh piece of crap (I learned the hard way when trying to record a beautiful 100 year old instrument that had been played in some of the finest halls in Europe, played by a very fine player - which due to my less than ideal room and less than ideal selection of mics, sounded like crap). I've since improved both my room and my mic locker!!!!
I couple of years ago, I bought a cheap violin and taught myself how to play some country licks - but I still find it better (for my applications) to trigger samples with a keyboard).