Xen has a very very very old Mellanox driver installed:
ethtool -i eth2
version: 3.1-1.0.4 (Oct 2015)
At this time, drive is already at version 4.1-184.108.40.206, which is dramatically different.
Please integrate new driver because the old one with new cards firmware is not supported anymore and give many performance issue due to compatibility issue with firmware