One problem of virtual server based on qemu KVM is that IO operations are slow in some cases.
The most common reason is that virtual disks are stored on RAID and virtual is using default HDD configuration.
It is recommended to turn off cache and set io operations to native:
<driver name='qemu' type='raw' cache='none' io='native'/>
There is very good article at serverfault. You can find there further explanations.