Have you checked known issues in the release notes ? I see items 58 and 59 for xen: http://www.mellanox.com/related-docs/prod_software/Mellanox_OFED_Linux_Release_Notes_2_2-1_0_1.pdf
Please share more details: screenshot, kernel version, HCA FW version and type. You can run also mstflint utility (http://www.mellanox.com/downloads/MFT/mft-3.6.0-24.tgz) like "mstflint -d 83:00.0 -qq q" to get more details on card (where 83:00.0 is pci device id for Mellanox card you can get with "lspci | grep Mellanox")
Thanks