I finally got MythTV running under Xen last night. It was quite a bit more work to get it running under Debian than Ubuntu which works out-of-the box, but Ubuntu has problems installing using debootstrap.
There's one major problem though: Xen seems to choke on large volumes of I/O going across the PCI bus. The whole machine locks up. So I think I'll end up having to run Myth inside dom0, though at least I can still run the other stuff I want for this server in various domUs.
I'm buying a couple of 300 gig hard drives off Moz, which should give me lots of disk space for recording telly. Just in time too, as the non-ratings period has ended and there's the occasional good show on now.
Last night I also bought one of these digital telly cards. They're the cheapest ones going and, to boot, they can also record analogue simultaneously. They're actually Pinnacle 300i cards and fully supported under Linux. They're clearance items from Dickies so get in quick if you want them. I think I might buy another as that would let me record five channels if you include analogue and my USB stick DVB thingy.
So the job over the weekend (apart from going to Good Vibrations) is to get Myth running in dom0 and get the diskless front end going.