Xen - Connect two VMs via Virtual Serial Port
I have recently been working on debugging The Windows kernel.
For the version of Windows I am using (7 professional / 32-bit), the easiest way to debug the kernel is via serial port.
In VirtualBox this is easy, as VirtualBox provides robust serial port options that allow the user to specify a unix socket to use as a virtual serial cable. If the unix socket doesn't exist before the VM is booted VirtualBox, when booting the server VM, will create the unix socket on the filesystem. The client VM can then connect to the unix socket by specifying the same path in the VirtualBox serial port settings.
However, for my purposes it became necessary to use Xen as the hypervisor. I started by installing and configuring Xen on an Ubuntu server, then spinning up two Windows HVM guest VMs.