Virtual Machine Instances
An instance is a running virtual machine, in addition to some data the cloud maintains to help track ownership and status. The cloud manages a large pool of hardware that can be used to create running instances from images. The virtual machine includes a copy of the image that it instantiates and the changes that it saves while it runs. The instance also includes virtualizations of the different hardware that it needs to run, including CPUs, memory, disk, and network interfaces. The cloud manages a pool of hypervisors that can manage the virtual machine instances. However, as a user of the cloud, you do not need to worry about the hypervisors. In fact, the hypervisor you are using—KVM, Xen, VMWare, or any other—makes no difference.
When you delete an instance, that hardware can be reused. The cloud scrubs your hard disk before doing so, to make sure that the next user of the hardware finds no traces of previous data.