I managed to install Xen+NetBSD on a spare machine. I can boot the kernel 
(xen12netbsd.gz) from the LiveCD in dom0 and from there I can create additional 
domains and boot them with a locally compiled NetBSD/xen kernel.

If I try to boot domain0 with a locally build kernel (one that works fine in 
domain 1), xen produces the message "DOM0: Invalid guest OS image" and reboots. 
   The kernel config file is the one on sys/arch/xen/config/XEN, the only 
change is 'netbsd  root on wd0a type ffs' as grub does not pass boot parameters 
to the kernel.

Have I missed a step somewhere?

-- 
     - aew