By default (on Debian system) Xen uses ttyS0 as serial console, and that causes conflict with kernel module 8250.ko. I personally met this trying to setup ‘nut’ package for UPS handling. The solution was found on XenSource page. Actually you just need to add # xenkopt=xencons=ttyS16 line to /boot/grub/menu.lst, do ‘update-grub’ and reboot.