Following this instruction, I created a raw drive image with the command:
qemu-img create xubuntu.img -f qcow2 2GI used the option -m 750 to start the virtual machine (VM) with 750 Mb of RAM. Then I used the option -cdrom to use a file as a cdrom image and the option -boot d to say that the VM shall boot on a cdrom drive. The command:
kvm -m 750 -cdrom ../xubuntu-9.10-desktop-i386.iso -boot d xubuntu.img...starts the installation of Xubuntu on the virtual machine.