When you install Ubuntu in Xen with xen-create-image, the console is automatically handled. If you want to add a console to a stock-installed Ubuntu, add this file to /etc/init and call it hvc0.conf:

# hvc0 - getty
# This service maintains a getty on hvc0 from the point the system is
# started until it is shut down again.
start on stopped rc RUNLEVEL=[2345]
stop on runlevel [!2345]
exec /sbin/getty -8 38400 hvc0