I have several qemu disk images with some of them includes Xorg. All of the virtual machine runs quite fast under Ubuntu. But when I want to start same image under Debian like this:
$ qemu-system-x86_64 -m 1024 -hda vm.img
everything works but performance is too low.
What can be the main problem of this slowness while qemu versions are same?
Most of the performance hit comes from KVM.
KVM (Kernel Virtual Machine) is a Linux kernel module that allows a user space program to utilize the hardware virtualization features of various processors: http://wiki.qemu.org/KVM
To enable kvm virtualization support under qemu, you need to pass -enable-kvm option:
$ qemu-system-x86_64 -enable-kvm -m 1024 -hda vm.img
Or you can install qemu-kvm package with apt and run kvm script like that:
$ kvm -m 1024 -hda vm.img