I don't know too much about LLVM, but the JVM has a ton of advantages: it's pretty much universally available on all platforms, it uses a fairly trivial set of instructions that make learning the instruction set and using it fairly easy, there's a ton of information out there about it covering everything from GC algorithms to VM flags to bytecode instructions and memory models, and both the Sun and IBM versions have been heavily optimized over the years. It might not be the best fit for every language, but it's a pretty impressive platform to build a language on.