[libvirt] A difference between libvirt and execution in bash shell

Once I type my qemu command line in bash shell and execute it. Then I use ps -aux to list the qemu process, results indicates the user of the qemu vm is "root" (Of course I logged with root). However, if I use libvirt to start a qemu vm, the user of qemu become "qemu". Could you tell me something about the user "qemu"? Also, how does this difference happen? Thank you.