I also had a problem starting a Xen HVM domU after upgrading to trixie,
but I don't know if it is the same problem as Elliott.
Elliott, can you check these logs: /var/log/xen/qemu-dm-*.log*
If you have an error similar to this:
xen-qemu-system-i386: -device
rtl8139,id=nic0,netdev=net0,mac=XX:XX:XX:XX:XX:XX: failed to find
romfile "efi-rtl8139.rom"
then you need to install package ipxe-qemu, even if you don't boot your
VM from network.
In trixie, qemu-system-xen no longer depends on ipxe-qemu, which
reintroduces a previous issue:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1035676
