May I suggest the following patch (or one similar) to hopefully answer
a FAQ and save some xen users the time searching the mail archives.

Index: ddb.4
===================================================================
RCS file: /export/cvs/netbsd/src/share/man/man4/ddb.4,v
retrieving revision 1.93
diff -u -r1.93 ddb.4
--- ddb.4       25 Feb 2006 02:28:56 -0000      1.93
+++ ddb.4       5 Apr 2006 09:47:36 -0000
@@ -164,6 +164,8 @@
 \*[Lt]Esc\*[Gt]-\*[Lt]Shift\*[Gt]-D on serial console.
 .It x68k
 Interrupt switch on the body.
+.It xen
++++++
 .El
 .Pp
 The key sequence to activate


Thank you,
Jeff