Grabbing the output from the kernel is simple: Add “console=tty0 console=ttyS0″ to your kernel command line. Then cat ttyS0 on the other end. For more details, there’s the TDLP Remove Serial Console Howto

Getting the Xen hypervisor to do something similar was slightly trickier. Turns out you have to use minicom, or all you get is a string of control codes. After some help from ##xen on Freenode, I was able to find OpenSuse Wiki: How to capture Xen hypervisor and kernel messages using a serial cable.

I can now get Xen output – however it seems to block the kernel output, so it’s one or the other at the moment. Would be nice if I can find out how to get both (kernel and hypervisor) working at the same time.