HipHop took PHP code and basically turned it into a statically compiled C++ application. The resulting binary acted directly as a web server. It was pretty fast, but very limited.
HHVM is an actual JITing virtual machine for PHP. It's still huge (one of the project pages say that the binaries are nearly a hundred megs), but it's now a real-world reimplementation of PHP, capable of doing anything PHP can, including eventually being run by mere mortals with normal code.
HipHop took PHP code and basically turned it into a statically compiled C++ application. The resulting binary acted directly as a web server. It was pretty fast, but very limited.
HHVM is an actual JITing virtual machine for PHP. It's still huge (one of the project pages say that the binaries are nearly a hundred megs), but it's now a real-world reimplementation of PHP, capable of doing anything PHP can, including eventually being run by mere mortals with normal code.