I have a NetBSD 3.99.17 domU on Xen 3. Networking seems to work fine
but tcpdump is blind: it never displays anything.

The interface is:

xennet0: flags=8863<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        address: 00:16:3e:06:5b:17
        inet 172.19.1.33 netmask 0xffffff00 broadcast 172.19.1.255
        inet6 fe80::216:3eff:fe06:5b17%xennet0 prefixlen 64 scopeid 0x2

Another domU, a Gentoo/Linux can see its traffic with tcpdump so I do
not think it is a general Xen problem.

A tcpdump on the dom0 (a Debian/Linux) does show the packets to/from
the NetBSD machine.