I used to use the command line incantations to run QEMU and KVM, but recently I've moved over to using virt-manager because it so much simpler to use and caters for most of my configuration needs.
Virt-manager provides a very usable GUI and allows one to create, manage, clone and destroy virtual machine instances with ease.
|virt-manager view of virtual machines
Each virtual machine can be easy reconfigured in terms of CPU configuration (number and type of CPUs), memory size, boot options, disk and CD-ROM selection, NIC selection, display server (VNC or Spice), sound device, serial port config, video hardware and USB and IDE controller config.
One can add and remove additional hardware, such serial port, parallel ports, USB and PCI host devices, watchdog controllers and much more besides.
|Configuring a virtual machine
..so reconfiguring a test to run on a single core CPU to multi-core is a simple case of shutting down the virtual machine, bumping up the number of CPUs and booting up again.
By default one can view the virtual machine's console via a VNC viewer in virt-manager and there is provision to scale the screen to the window size, set to full size or resize the virt-manager window to the screen size. For ease of use, I generally just ssh into the virtual machines and ignore the console unless I can't get the kernel to boot.
|virt-manager viewing a 64 bit Natty server (for eCryptfs testing)
Virt-manager is a great tool and well worth giving a spin. For more information on virt-manager visit virt-manager.org