SFV stands for Simple File Verification.
Some of the downloaded files in the Net come bundled with a file that has an extension of .sfv -- this file is simply a verification file for the downloaded file itself.
SFV is basically a mathematical short checksum number based on the original file. When user downloads the original file and the .sfv file, user can use dedicated SFV check programs that use the SFV data to check the status of the downloaded file. If the SFV's checsum data doesn't match to the data that the validation program gets from the downloaded file, it typically means that the downloaded file is not complete or that the downloaded file has been corrupted during the transfer and needs to be downloaded again.
Related software tools
QuickSFV is a fast and free file verification program that supports file verification based on .sfv, .md5, .crc, .par and variety of other formats.