ESXi hosts can reclaim memory from virtual machines.

A host allocates the amount of memory specified by a reservation directly to a virtual machine. Anything beyond the reservation is allocated using the host’s physical resources or, when physical resources are not available, handled using special techniques such as ballooning or swapping. Hosts can use two techniques for dynamically expanding or contracting the amount of memory allocated to virtual machines.

ESXi systems use a memory balloon driver (vmmemctl), loaded into the guest operating system running in a virtual machine. See Memory Balloon Driver.

ESXi systems page from a virtual machine to a server swap file without any involvement by the guest operating system. Each virtual machine has its own swap file.