I've filled a pr the past month, some of the recently merged branches broke the dom0 xen kernel, here is the bug report: http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=45062 Take a look to see if it's related.
Just a heads up, this PR was finally fixed yesterday. If you update to today's -current, you should be able to use the resulting dom0 kernel.
+j