If you ever want to get to the grub menu while using pygrub - there is a really simple way of doing that. Just add :
bootargs="-i"
into the /etc/xen/{domU config file}. And the next time you start the VM, it will bring up the grub menu. Quite handy when you need to recover the root passwords or to be able to use a different init script as a one off.
Essentially, that bootargs will pass in parameters to pygrub during the domU boot phase. To get a list of all the possible options you can pass in with bootargs, try this: pygrub --help. You should get output like this :
# pygrub --help
Usage: /usr/bin/pygrub [-q|--quiet] [-i|--interactive] [--output=] [--kernel=] [--ramdisk=] [--args=] [--entry=] image