CentOS 6 x86-64 guest
The first graph shows CentOS 6 guest disk speed with and without the paravirtualized driver:
Native performances are included for reference only. We can see that para-virtualized disk driver provide a good speedup versus the standard virtualized IDE controller. Anyway, both approaches are far behind the native scores.
Net speed now:
In this case the paravirtualized network driver makes an huge difference: while it can't touch native speed, it is way ahead of the virtualized E1000 NIC adapter. The RTL8139 was benchmarked for pure curiosity, and it show a strange behavior: while output speed is in line with NIC speed (100 Mb/s), input speed is much higher (~400 Mb/s). Strange, but true.
While host CPU load is lower on the full virtualized NICs, it is only because they deliver much lower performance. In other word, the Mb/s per CPU load ratio is much higher on the para virtualized network driver.