It is obvious that the paravirtualized drivers are an important piece of the KVM ecosystem. While the fully virtualized drivers are quite efficient and the only way to support a large variety of guest OSes, you should really use a paravirtualized driver if available for your guest virtual machine.
Obviously performance are only part of the equation, stability being even more important. Anyway I found the current VirtIO drivers release very stable, at least with the tested guests.
In short: when possible, use the VirtIO paravirtualized drivers!
Have a nice day!