vSphere uses several memory techniques: Ballooning, memory compression and transparent page sharing. Last level of managing memory overcommit is hypervisor swapping (not desired).
A good example is a scenario where several virtual machines are running instances of the same guest operating system, have the same applications or components loaded, or contain common data. In such cases, a host uses a proprietary transparent page sharing technique to securely eliminate redundant copies of memory pages. As a result, higher levels of over-commitment can be supported.