I just installed 11.1 from DVD. I set up Xen, and installed a Centos 5.3 domU. Everything is fine.
I then made the mistake of installing updates. That broke Xen quite dramtically. The system wouldn’t boot (“no operating system”).
To recover, I booted from the DVD and chose the recovery system. I then reinstalled the boot blocks:
root (hd0,5) would be the entry from your menu.lst if you can read it. This corresponds to /dev/sda6, as grub numbers from 0.
Then, I was unable to get it to activate Xen. Install hypervisor claims to work, but doesn’t add the Xen entry to menu.lst.
I added the following
module /boot/vmlinuz-xen resume=/dev/sda5 splash=silent showopts vga=0x31a
resume should be your swap area. There’s also repair=1 in the normal boot entry. I don’t know what it does or whether it should be used here.