Good question, since I don't know it either. I can only assume that it stands for fraktionslose (independent/non-party) members. Then again, there should be two of them.
EDIT: Alright, I checked it. The numbers given above should be correct, except that there are two independent members (list under "AFV"? there).
Thank you. With this information I've found out what AFV stands for. It is the new party (movement?) of Bundestag member Henry Nitzsche (ex-CDU), the "Wählervereinigung Arbeit, Familie Vaterland":
http://www.arbeit-familie-vaterland.de/