Don't get the HVX. Get a Sony AX11 or a X70. The HVX is an out of date camera shooting to expensive media, and getting a fisheye for it is a pain in the aris unless you don't mind Opteka, and still, the camera is heavy and crap in low light.
If you still wanna get it, just get an Opteka/Kapkur 72mm fisheye, and get a step down ring from 82mm to 72mm and you'll be good to go. Save the few grand you'd spend on a Xtreme and do something nice with your girlfriend/boyfriend, or buy your mum a present. Buy some drugs I don't know. Good luck