Got a shiny new Dell Optiplex 740 (AMD 64 X2 goodness) that I wanted to run request-tracker in a VM on. Installed ubuntu gutsy fine, Installed Xen fine, just wouldn't correctly boot any domU's to completion. I even tried out KVM seeing as it's new hardware that has the 'svm' flags in /proc/cpuinfo. That failed miserably at the modprobe kvm_amd stage with kvm: disabled by bios. Most odd as there wasn't a bios option for v12n. However I (after much hassle finding a bootable flopy) upgraded from 1.1.3 to 1.2.2 BIOS and lo - xen 'Just Works'
it's actually scarily easy to do - "xen-create-image --hostname whatever" and you're off :-)
More postings as I get to grips with this. Initial impressions are good.