XEN4 and Slackware dom0
=======================
root@dom0:/etc# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
88.191.123.0    *               255.255.255.0   U     0      0        0 eth0
default         88.191.123.1    0.0.0.0         UG    0      0        0 eth0

root@dom0:/etc# tcpdump  -i vbsd64.0
tcpdump: WARNING: vbsd64.0: no IPv4 address assigned
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on vbsd64.0, link-type EN10MB (Ethernet), capture size 65535 bytes
23:58:20.900681 IP mx3.nethence.com > mx3.nethence.com: ICMP echo request, id 45765, seq 0, length 64

root@dom0:/etc# tcpdump  -i dummybr0
tcpdump: WARNING: dummybr0: no IPv4 address assigned
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on dummybr0, link-type EN10MB (Ethernet), capture size 65535 bytes
23:58:07.132389 IP mx3.nethence.com > mx3.nethence.com: ICMP echo request, id 23860, seq 0, length 64


NetBSD guest
============
# arp -a
? (88.191.123.229) at 78:65:6e:6e:65:74:30:00:00:00:fa:ce on xennet0 permanent
# netstat -r -f inet
Routing tables

Internet:
Destination        Gateway            Flags    Refs      Use    Mtu  Interface
88.191.123.229     78:65:6e:6e:65:74  UHLS        0        3      -  xennet0
88.191.231.159/32  link#1             UC          0        0      -  xennet0
127/8              localhost          UGRS        0        0  33648  lo0
localhost          localhost          UH          1       20  33648  lo0

tcpdump on guest's side doesn't see pings arrive from the dom0