I am running a 64-bit host and was running a 32-bit XPguest which was crawling no matter what I was doing with it,in fact it frequently used 100% CPU power while having no real work load. I experienced with this other guests and wondered why KVM was slow compared to say Xen or Virtualbox.
This is because you need to use the "-cpu" flag.
For a full list of options run:
/usr/libexec/qemu-kvm -cpu ?
x86 ........