Besides what Trevor wrote, if e1000 works I would recommend using it. If I remember correct creating a new VM in ESXi when using RHEL5 as OS suggests e1000 NICs. [by tigalch]
I am running a low latency demanding appliction on a VM in VSphere 5.1.
The TCP/IP transfer rate is 124 MBit.
I have an intermittent problem related to the network dropping and then slowly increasing to full speed again.
I have attached a diagram of data gathered from the performance of the ESXi.
When start VMs with -enable-kvm I get following error message in my qemu.log and the VM does not boot after this:
Warning: nic e1000.0 has no peerWarning: nic e1000.1 has no peerWarning: nic e1000.2 has no peerWarning: nic e1000.3 has no peerWarning: nic e1000.4 has no peerWarning: nic e1000.5 has no peerKVM internal error.