Stange. That'd mean that the XenServer product is using a Dom0 that doesn't play nice with the hardware. I checked my inventory, I have two HP servers running Xen (not XenServer, just Xen), a DL180 G6 and a DL360 G6, and they are doing just fine. They are nearly identical setups; Xen 4.6 and Xen 4.8, but both Debian 8 as a Dom0 host, pfSense 2.3.2-p1 as firewall and a bunch of DomU's, all Linux (mix of Debian and Fedora). I'm using the internal NIC's on both of them, and pulling 100Mbit up/down. (branch office uplinks)