linux KVM guests network performance is slower than host

i have a KVM linux based virtual machines, as the network performance of the host is running perfect with 1 Gbps (on the internal network) and using iperf tools it show that the network is running perfect on the host, but guests have slow network performance as iperf result was so strange as it is not 100 Mbps and is not 1 Gbps, it's between 350 and 500 Mbps maximum. This is the result of runnin iperf -c servername -P 15:

[ ID] Interval       Transfer     Bandwidth[ 17]  0.0-10.0 sec  68.6 MBytes  57.5 Mbits/sec[ ID] Interval       Transfer     Bandwidth[  4]  0.0-10.0 sec  18.6 MBytes  15.5 Mbits/sec[ ID] Interval       Transfer     Bandwidth[  5]  0.0-10.0 sec  61.9 MBytes  51.9 Mbits/sec[ ID] Interval       Transfer     Bandwidth[ 11]  0.0-10.0 sec  16.0 MBytes  13.4 Mbits/sec[ ID] Interval       Transfer     Bandwidth[ 10]  0.0-10.0 sec  24.3 MBytes  20.4 Mbits/sec[ ID] Interval       Transfer     Bandwidth[ 15]  0.0-10.0 sec  20.2 MBytes  16.9 Mbits/sec[ ID] Interval       Transfer     Bandwidth[ 12]  0.0-10.0 sec  30.2 MBytes  25.3 Mbits/sec[ ID] Interval       Transfer     Bandwidth[  3]  0.0-10.0 sec  36.2 MBytes  30.3 Mbits/sec[ ID] Interval       Transfer     Bandwidth[  8]  0.0-10.0 sec  23.6 MBytes  19.8 Mbits/sec[ ID] Interval       Transfer     Bandwidth[  9]  0.0-10.0 sec  24.2 MBytes  20.3 Mbits/sec[ ID] Interval       Transfer     Bandwidth[ 14]  0.0-10.0 sec  31.0 MBytes  25.9 Mbits/sec[ ID] Interval       Transfer     Bandwidth[  7]  0.0-10.0 sec  16.6 MBytes  13.9 Mbits/sec[ ID] Interval       Transfer     Bandwidth[  6]  0.0-10.0 sec  18.8 MBytes  15.7 Mbits/sec[ ID] Interval       Transfer     Bandwidth[ 13]  0.0-10.0 sec  13.3 MBytes  11.2 Mbits/sec[ ID] Interval       Transfer     Bandwidth[ 16]  0.0-10.0 sec  29.6 MBytes  24.8 Mbits/sec[SUM]  0.0-10.0 sec    433 MBytes    363 Mbits/sec

I tested different network models and the best result was with the virtio model any one please can help to get the full performance of network and be 1 Gbps.

I would like to add that when i execute the command brctl show I got the fbridge name

bridge id       STP enabled interfacesbr0     8000.d067e5fb7dec   no      eth0                            vnet0                            vnet1                            vnet2br2     8000.d067e5fb7dee   no      eth1virbr0      8000.000000000000   yes

can this affect the network speed of the guest?