I tried adding it to my config file that #includes XEN3_DOM0; com0 doesn't show up at probe time. However -- and for reasons I don't understand -- configuring ttys to turn on /dev/console rather than /dev/tty00 is working properly, in that it's speaking to the com0 serial port. (In a message exchange a few months ago, tjls had suggested the opposite -- that doesn't work here.)
I don't have com0 defined in my XEN2 Dom 0. My entire Dom 0 works through the serial port with just this in the grub config: default=0 timeout=10 serial --unit=0 --speed=115200 --word=8 --parity=no --stop=1 terminal --timeout=10 serial console title Xen 2.0 / NetBSD (serial) root(hd0,0) kernel (hd0,a)/xen.gz dom0_mem=73728 com1=115200,8n1 module (hd0,a)/netbsd-XEN0 ro console=ttyS0 ... and of course the following in my ttys file: console "/usr/libexec/getty std.115200" vt100 on secure