Running a KVM virtual machine on your desktop with the default screen output is fine if you're using it on your PC for desktop use. However, for running KVM guest machines in a server setting where you may have many virtual machines running concurrently, the best way to run the guest machine(s) is headless with a vncserver attached to the video console. To do this, run qemu/kvm with the -vnc option. For example
qemu-system-x86_64 -hda windows.img -m 384 -vnc :1
Note the space between -vnc and :1. This will start the virtual machine without any video output to your desktop screen so that it's now running as a headless daemon. To get a video console to your running machine, open up a vnc client and connect to the ip address of the kvm host with :1 at the end of the address. For example, if the ip address of the kvm host is 192.168.1.1 you will connect your vnc client to 192.168.1.1:1. If you're going to start any more headless machines concurrently then increment the display numbers ie :2, :3, :4 etc. This is a simple procedure but I'm sure there are some newbies out there who aren't very familiar with qemu/kvm's many options.