I had this issue with KVM (CentOS EL 6.4) so I ran a benchmark to find out what was happening. Turns out my guest VM was running only 15% as fast as the host (bare metal OS). I had checked the prerequisites of kvm and verified my Intel processor has the VT-x (vmx) [AMD calls it svm].
cat /proc/cpuinfo | grep vmx
Why then the slow performance?? Turns out that I needed to enable VT-x in the BIOS of the system (older machine circa 2006). To verify that this was the problem, I checked to see if kernel module
kvm_intel was running:
lsmod | grep kvm
Turns out that it was not running with
kvm_intel. When I had installed my virtual machine, libvirt defined the
domain type as
qemu. But this should be type
kvm. You can check:
virsh edit your_vm_guest_domain
... check the line at the top ...
That was libvirt telling me it didn't detect the
kvm_intel presence on my system and defaulted to plain qemu emulation.
When I enabled the VT-x in the BIOS, I found that I could load the
kvm_intel module and then changed the
<domain type='kvm'>. I executed the same benchmark again and found virtually no difference in speed between the guest VM and the host OS.