To run a kernel directly in
kvm, use the
qemu-system-x86_64 -kernel /boot/vmlinuz-3.2.0-2-amd64
Virtualbox may have something similar.
If you really want it installed on a hard drive image, it's a bit harder. You need a bootloader installed, which may also require a partition table. Something like SYSLINUX or EXTLINUX could be straightforward, or you could just go straight ahead to what most real systems use and install GRUB. For GRUB, the instructions in Linux From Scratch will be helpful, as will the GRUB documentation itself.