To create new Centos 6 guest from command line run this:

qemu-kvm -m 512 -drive file=/home/vit/kvm/centos,if=virtio -cdrom /home/vit/kvm/CentOS-6.4-x86_64-minimal.iso -net nic -net tap,ifname=tap0,script=no,downscript=no -smp 4 -cpu host -boot d -daemonize

-m MB
-drive disk image
-net network
-boot d boot from cdrom