ART is a new Android runtime being introduced experimentally in the 4.4 release. This is a preview of work in progress in KitKat that can be turned on in Settings > developer options. This is available for the purpose of obtaining early developer and partner feedback.
Important: Dalvik must remain the default runtime or you risk breaking your Android implementations and third-party applications.
Two runtimes are now available, the existing Dalvik runtime (libdvm.so) and the ART (libart.so). A device can be built using either or both. (You can dual boot from Developer options if both are installed.)
dalvikvm command line tool can run with either of them now.
See runtime_common.mk. That is included from build/target/product/runtime_libdvm.mk or
build/target/product/runtime_libdvm.mk or both.
PRODUCT_RUNTIMES variable controls which runtimes
are included in a build. Include it within either
build/target/product/core_minimal.mk or build/target/product/core_base.mk.
Add this to the device makefile to have both runtimes
built and installed, with Dalvik as the default:
PRODUCT_RUNTIMES := runtime_libdvm_default
PRODUCT_RUNTIMES += runtime_libart