I have it working with Xen just fine as well. Works with onboard but also with plug-in PCIe NIC's. Using a 4-port Intel-based one is doing just fine.
Try to see if pciconf sees the passed cards at all, it should say something like:
[2.3.2-RELEASE][john@fw-1-prod]/home/john: pciconf -l | grep igb
igb0@pci0:0:8:0: class=0x020000 card=0x12a18086 chip=0x150e8086 rev=0x01 hdr=0x00
igb1@pci0:0:9:0: class=0x020000 card=0x12a18086 chip=0x150e8086 rev=0x01 hdr=0x00
igb2@pci0:0:10:0: class=0x020000 card=0x12a18086 chip=0x150e8086 rev=0x01 hdr=0x00
igb3@pci0:0:11:0: class=0x020000 card=0x12a18086 chip=0x150e8086 rev=0x01 hdr=0x00