Comment # 28 on bug 919154 from Thomas Blume

Some more findings.
On the guest, I can see that:

/sys/devices/xen/vif-0/net/eth0/statistics/tx_packets

is still increasing when I do a ping from the guest to the host after the
network hang.
There are no dropped packets visible.

On the host, I can see that:

/sys/class/net/vif1/statistics/tx_dropped

is increasing (vif1 is the guest interface):

bridge name    bridge id        STP enabled    interfaces
br1        8000.feffffffffff    no        vif1

The host is running kernel 3.16.7-21-xen from 13.2 that doesn't show the bug
when installed on the guest.
So, it seems that the guest kernel is dropping packages coming in from the host
but still sends out packages to the host.
This is a pretty weird behaviour.