[Fedora-xen] 32 bit F7 HVM hangs under 64 bit F6 host

[tom horsley att net (Tom Horsley)]

All my attempts to install a Fedora 7 HVM under the x86_64 Fedora 6
host I have have gone into 100% cpu use hangs during the install.

Today I tried something new - I installed Fedora Core 6 as an HVM,
then built a repo from the F7 dvd image and updated all the packages.

That actually ran, but it only works if I boot the old Fedora Core 6
kernel. Any attempt to boot any F7 kernel I've tried gets far enough
to print:

PCI: PIIX3: Enabling Passive Release on 0000:00:01.0

then it is hung in the same ol 100% cpu usage state.

Anyone know what is happening right there (or right after there)?
Any good kernel boot parameters to try?