I tried to setup a separate domain to run ivtv and the mythbackend, but it apparently wasn't meant to be. After trying various incantations of "swiotlb=force" in both dom0 and domU, thing still didn't work. I eventually got the devices passed through (PVR-500 - essentially two PVR-250's on a PCI bus) and they would initialize, but they would lock up with DMA errors when I would try to cat /dev/video0. Unfortunate.
I finally also got NFS setup on the file server, but idmapd doesn't seem like it works quite right - at least with the NFS server that comes with fc6. While I would be able to see the correct group names even though the group ids were different, I wouldn't be able to write to the directory that was chmod 2770 (and the user was in the group). After syncing the GIDs on the client and server everything worked fine. Unfortunate.
I finally just got mythbackend setup on Thor with the PVR-500 setup there, disabled Xen, and setup a mythfrontend on my HTPC. I copied over the asound.state file (sound card mixer settings), lircd.conf and lircrc (IR remote control config), and configured the frontend to use 720x480 in the recording profile (stupid ghosting/flickering problem) everything seems to be working fine. It's nice to have a working PVR after a couple weeks without. Nice.