tcpdump on vif:

 1, length 64
15:24:23.717143 IP (tos 0x0, ttl 64, id 28419, offset 0, flags [DF], proto ICMP (1), length 84)
    192.168.100.122 > arn02s05-in-f24.1e100.net: ICMP echo request, id 1560, seq 2, length 64
15:24:24.725266 IP (tos 0x0, ttl 64, id 28420, offset 0, flags [DF], proto ICMP (1), length 84)
    192.168.100.122 > arn02s05-in-f24.1e100.net: ICMP echo request, id 1560, seq 3, length 64
15:24:25.733269 IP (tos 0x0, ttl 64, id 28421, offset 0, flags [DF], proto ICMP (1), length 84)
    192.168.100.122 > arn02s05-in-f24.1e100.net: ICMP echo request, id 1560, seq 4, length 64
15:24:26.741240 IP (tos 0x0, ttl 64, id 28422, offset 0, flags [DF], proto ICMP (1), length 84)
    192.168.100.122 > arn02s05-in-f24.1e100.net: ICMP echo request, id 1560, seq 5, length 64
15:24:27.749122 IP (tos 0x0, ttl 64, id 28423, offset 0, flags [DF], proto ICMP (1), length 84)
    192.168.100.122 > arn02s05-in-f24.1e100.net: ICMP echo request, id 1560, seq 6, length 64

tcpdump on local NIC on physical Xen host when pinging from the guest:
15:31:47.568468 ARP, Ethernet (len 6), IPv4 (len 4), Request who-has 192.168.100.1 tell 192.168.100.122, length 28
15:31:47.568612 ARP, Ethernet (len 6), IPv4 (len 4), Reply 192.168.100.1 is-at 78:2b:cb:67:c1:01, length 46
15:32:10.592473 ARP, Ethernet (len 6), IPv4 (len 4), Request who-has 192.168.100.1 tell 192.168.100.122, length 28
15:32:10.592572 ARP, Ethernet (len 6), IPv4 (len 4), Reply 192.168.100.1 is-at 78:2b:cb:67:c1:01, length 46

tcpdump on local NIC on physical Xen host when pinging from the "router":
15:33:38.242955 IP (tos 0x0, ttl 64, id 22101, offset 0, flags [DF], proto ICMP (1), length 84)
    192.168.100.1 > 192.168.100.122: ICMP echo request, id 31525, seq 1, length 64
15:33:38.243245 IP (tos 0x0, ttl 64, id 63203, offset 0, flags [none], proto ICMP (1), length 84)
    192.168.100.122 > 192.168.100.1: ICMP echo reply, id 31525, seq 1, length 64
15:33:39.241915 IP (tos 0x0, ttl 64, id 22102, offset 0, flags [DF], proto ICMP (1), length 84)
    192.168.100.1 > 192.168.100.122: ICMP echo request, id 31525, seq 2, length 64
15:33:39.242145 IP (tos 0x0, ttl 64, id 63204, offset 0, flags [none], proto ICMP (1), length 84)
    192.168.100.122 > 192.168.100.1: ICMP echo reply, id 31525, seq 2, length 64
15:33:40.240959 IP (tos 0x0, ttl 64, id 22103, offset 0, flags [DF], proto ICMP (1), length 84)
    192.168.100.1 > 192.168.100.122: ICMP echo request, id 31525, seq 3, length 64
15:33:40.241238 IP (tos 0x0, ttl 64, id 63205, offset 0, flags [none], proto ICMP (1), length 84)
    192.168.100.122 > 192.168.100.1: ICMP echo reply, id 31525, seq 3, length 64