In "Red Hat Enterprise Linux 6. Virtualization Host Configuration and Guest Installation Guide" in chapter 10.1 "Installing the KVM Windows para-virtualized drivers" it says:
The virtio-win package contains the para-virtualized block and network drivers for all supported Windows guest virtual machines. The virtio-win package can be found here in RHN
Alas, RHN is a premium feature of Red Hat Enterprise Linux. Are virtio drivers available on CentOS?