HipHop compiles the script natively, while KittenDB seems to converts to C++ and then compiles it. KittenDB seems to have fewer dependencies, meaning it's likely to be less feature rich. This could be down to the fact that HipHop is actually a full stack, while this is just the interpretation layer.
I'm finding it difficult to find any more details because the documentation is in Russian.
I'm interested in the fact that both Facebook and the Russian equivalent independently started work on the same basic idea. It's almost like convergent evolution.
The original version of HipHop also compiled to C++. Very slow to compile C++ actually. HipHopVM is the one that compiles it natively, it's a successor to the original.
It requires additional dependencies because HHVM has to emulate most of the core PHP extension functionality to be compatible with PHP, for example 'mysql_query', etc.
I'm finding it difficult to find any more details because the documentation is in Russian.
I'm interested in the fact that both Facebook and the Russian equivalent independently started work on the same basic idea. It's almost like convergent evolution.