[ubuntu] Xen virtual machine stuck at "Booting from hard disk..."

I have a desktop with Windows XP installed in one partition (/dev/sda1) and Ubuntu 12.04 LTS Server installed in another (/dev/sda2). I can choose which operating system to run at boot time, and both work. In Ubuntu, I have installed Xen and Virtual Machine Manager. I can create a Windows XP virtual machine using a file as the virtual disk, and it works fine. However, when I try to create a Windows XP virtual machine using /dev/sda1 as the virtual disk, the virtual machine won't start; it's stuck at the message that says "Booting from Hard Disk..."

I know that the Windows XP operating system in /dev/sda1 is good because I can boot it directly (not as a virtual machine).

I know that Xen is working because I can create a virtual disk image, install Windows XP to it, and boot it from within Xen.

I know that I can attach a physical partition to a virtual machine, because I can take my existing virtual machine that boots from a virtual disk image, attach the physical partition as a second virtual disk, and boot it. From within the virtual OS, I can see the files on the second virtual drive (which is the physical partition /dev/sda1).

The only thing I can not do is boot a virtual machine that uses the physical partition /dev/sda1 as the virtual boot device. The virtual machine definition is exactly the same -- I just remove the virtual disk image and attach the physical partition, and the virtual machine will not boot.

I even tried to boot into Windows XP directly (not through Xen) and configure it so that it gives me a boot menu when it starts up, so that I can choose to boot into Safe Mode if necessary. When I try to boot it through Xen, I don't even get to that boot menu.

What else can I try to get this working?