Hi, I've installed xen in my computer on debian(lenny). And I have installed ubuntu 9.10 in a separate partition. So what I want to know is can I boot ubuntu 9.10 as a virtual machine from inside debian using xen ?

I don't believe you can do this as Xen is going to look for a file that represents the hard disk in the settings. What I usually do in this situation would be to get rid of the Ubuntu second partition, mount that as a drive to store Xen images, and run Ubuntu as a virtual machine.

You *might* be able to do this if you installed a paravirtualized kernel in ubuntu and booted it in PV mode instead of FV mode (PV doesn't technically require a full disk). You'd also have to make sure that you passed the ubuntu partition to the VM as a partition instead of as a full disk. For example, your disk line might look something like this:

disk = [ 'phy:/dev/sda3,sda3,w' ]

However, even with these changes you'd probably still run into problems with configuration settings for video, network, fstab, etc., that will make this a less than ideal thing to do.