Fedora xen - Jan 2007 - Poor Network Performance between dom-0 and dom-u

Hi,
i have problems concerning the network performance between
dom-0 and a dom-u host. Messurement using iperf show

[  4]  1.0- 2.0 sec  7.85 MBytes  65.8 Mbits/sec

Network performance between dom-0 and an external maschine
shows gigabit speed.

[  5]  0.0- 1.0 sec    112 MBytes    938 Mbits/sec

i am using Fedora core 6 on the dom-0 host and redhat 4.4 on the
dom-u guest. Dom-u is fully virtualized. (Dual Xeon 5150 2.66GHz.)
Any hints that could help to improve the performance are greatly 
appreciated.
--
Harald