Comment 47 for bug 144631

I don't know if this will help since it sounds a bit different:

We had some Ubuntu booting problems before (run manual build xen on centos). We fixed this by building a new initrd in the domU and copy that initrd to dom0 and use it in the config file instead of the supplied one.

you can build a new initrd with the following command:

example: `mkinitramfs -o /boot/initrd-2.6.18-xen-feisty.img 2.6.18-xen`

you don't have to start the domu, you can also build it with a chroot to the filesystem.

maybe its worth a try.