Edit your config file in /etc/xen/foo.cfg and add the line
extra       = 'console=hvc0 xencons=tty'
E.g.
kernel      = '/boot/vmlinuz-2.6.32-5-xen-amd64'
ramdisk     = '/boot/initrd.img-2.6.32-5-xen-amd64'
extra       = 'console=hvc0 xencons=tty'
vcpus       = '1'
memory      = '2048'