If you followed installer defaults, then /boot is ext4 (strike 3).
Your Xen version won't support ext4 in boot partition.
Its pretty easy to upgrade your Xen to 3.42 by pointing Yum to this repo:
http://www.gitco.de/linux/x86_64/centos/5/(watch the arch).
Best bet is to add the new *.repo from gitco,upgrade Xen, then try the PV install again and use ext3 in the /boot partition.
You will still have trouble with pygrub choking on the grub2 *.cfg file.
An option is to get hold of pv-grub for your arch and use that at first boot.