HHVM (HipHop Virtual Machine) is an open source virtual machine for running PHP and Hack code. HHVM is developed and maintained by Facebook.
HHVM differs significantly from other PHP machines because of its JIT compilation. HHVM compiles PHP code into intermediate byte code and then directly into commands and statements on 64-bit machines. This allows to optimize performance and achieve better performance.
The HHVM virtual machine is fast and powerful, so of course it is very demanding on resources (like many other VMs such as the JVM). HHVM requires more RAM and CPU resources than other, lighter PHP interpreters (e.g. PHP-FPM).