I'm trying to build a dom0 system (4.0beta2) with a serial console.
The standard XEN3-DOM0 doesn't include a com port; however, it 
includes XEN2_DOM0, which defines ISA com ports.  My machine is (and
should be) ACPI, which is supported by the XEN3-DOM0 config file.
Accordingly, I added 

attimer*        at acpi?                # AT Timer
com*            at acpi?                # Serial communications
interface #fdc*            at acpi?                # Floppy disk
controller joy*            at acpi?                # Joystick/Game port
lpt*            at acpi?                # Parallel port
mpu*            at acpi?                # Roland MPU-401 MIDI UART
#npx*            at acpi?                # Math coprocessor
pckbc*          at acpi?                # PC keyboard controller
pcppi*          at acpi?                # AT-style speaker sound
wss*            at acpi?                # NeoMagic 256AV in wss mode

to my config file, plus

no com1 at isa? 
no lpt0 at isa?
no pckbc0 at isa?

When I boot, though, I get this:

com0 at acpi0 (PNP0501-1)
com0: io 0x3f8-0x3ff irq 4
com0: com probe failed
com1 at acpi0 (PNP0501-2)
com1: io 0x2f8-0x2ff irq 3
com1: ns16550a, working fifo

I don't know why com0 is failing, unless perhaps it's because there's a
cable going to it.  com1 is just a header on the motherboard.  (I also
can't figure out how to configure the floppy, since including get gets
link-time errors about a missing interrupt routine, but since I doubt
I'll ever actually use a floppy on this machine I don't really care.)


		--Steve Bellovin, http://www.cs.columbia.edu/~smb