After getting along with the old kvm (installed with the apt-get install) for long, I'm trying to build kvm from the latest source code. However, it seems that the organization of the kvm source code is not so tidy and clean, many packets in sourceforge.net/projects/kvm/files/ , including qemu-kvm, kvm-kmod and kvm etc..
Assuming that the kvm source code in the kernel source tree always get updated with each release, what I need to do is to install the latest linux kernel and qemu-kvm. After that there is only qemu-system-x86_64 command instead of kvm. Who knows the difference between the two commands: kvm and qemu-system-x86_64 ? When a guest image was started with qemu-system-x86_64, is it actually using KVM as its hypervisor the same as with kvm command?