It would be sensible for hypervisors to require IOMMU for all PCI passthrough, but it looks like Xen allows it without IOMMU for paravirtualised guests:
"VT-d Pass-Through is a technique to give a domU exclusive access to a PCI function using the IOMMU provided by VT-d. It is primarily targeted at HVM (fully virtualised) guests because PV (paravirtualized) pass-through does not require VT-d (altough it may be utilized too)."[1]
"VT-d Pass-Through is a technique to give a domU exclusive access to a PCI function using the IOMMU provided by VT-d. It is primarily targeted at HVM (fully virtualised) guests because PV (paravirtualized) pass-through does not require VT-d (altough it may be utilized too)."[1]
[1] = http://wiki.xenproject.org/wiki/VTd_HowTo