Xen FAQ Guest Conversion
How do I convert a Centos HVM Guest to a PV Guest?
Creating a Centos HVM domU with working PV drivers : http://pastebin.com/fb6fe631 Converting HVM guest to PV guest : http://pastebin.com/f6a5022bf
If you follow both parts correctly you should have a working PV domU. If anything goes wrong during conversion process, you should still be able to boot the previous HVM domU config if you select the non-xen kernel (second entry) from grub menu.list.