The mode “Copy-On-Write”, often referred by the acronym COW, is available on some formats of virtual machine disk as QCOW2.
Specifically, when using the COW mode, no changes are applied to the disk image. All changes are recorded in a separate file preserving the original image. Several COW files can point to the same image to test several configurations simultaneously without jeopardizing the basic system.
QEMU / KVM allows to incorporate changes from a COW file to the original image.