What are the para virt guest?  If they are RHEL4, then this is a known bug and will be fixed in RHEL4.7.  There is a bugzilla with a fixed kernel for the RHEL4 guest if need be.  Sorry, but I don't have it handy and I will try to post it once I get into work tomorrow.  However, doing a tcpdump and then sshing between the two will show you the bug within the tcpdump capture.  There are bad check sums which cause the packets to not correctly get from Host A to Host B on the same dom0.

 

       -- Robin