[ubuntu] ubuntu 10.04 kvm guest's network speed slow

i have a host is ubuntu 10.04 64bit and installed a kvm virt-manager
the problem is when testing the network speed (using iperf) between the host the any network device i got the result perfect (1 GB/s) as the network card interface is 1 GB/s
but testing the network speed between the guest and any network device even the host itself , i got the result is betwen 400 MB/s and 550 MB/s

although i copied the virtual machine to another host rhel 5.3 64bit i got the full network speed and network speed was working perfect.

can any one guess what may be the problem , you help is highly appreciated and welcomed