Dalvik VM has been the default VM since first Android versions.
Google released the new ART runtime in KitKat 4.4.
AndroidPolice says it would increase app performance and battery life. Unfortunately, there's almost no technical information about ART. There's just this page.
From what I understood, Dalvik VM translates dex bytecode into machine instructions in runtime. ART precompiles dex code into native instructions, so that it eliminates runtime translation step.
With respect to this: