[compsciphd]

Not really "out of band", but many VMs allow you to setup a serial console, which is sort of that, albeit with a login, but in reality, could create one without one, still have to go through hypervisor auth to access it in all cases, so perhaps good enough for your case?

[blipvert]

Indeed, easy enough to get a serial device on Xen.

Another possibility could be to implement a simple protocol which uses the xenstore key/value interface to pass messages between host and guest?

[b112]

You can launch KVM/qemu with screen + text console, and just log in there. You can also configure KVM to have a VNC session on launch, and that ... while graphical, is another eye into the console + login.

(Just mentioning two ways without serial console to handle this, although serial console would be fine.)