CentOS virt - Oct 2007 - Initrd for Domu

I am using a DomU CentOS 5 installed on a LVM file.

I have kernel files in Centos DomU (/boot) and I am using pygrub for 
booting.

Using default initrd file DomU don't boot
Making a new initrd file using --preload xebblk --preload --xennet DomU 
boot ok.

Is this correct?

If is correct, how can I automate initrd production when I get a kernel 
update?

Regards

Franco Spinelli