Dalvik VM

The Dalvik Virtual Machine (DVM) was developed by Google and introduced with the first version of Android in 2008

Android applications written in Java or Kotlin are compiled into Java bytecode and then transformed into Dalvik bytecode, packaged in .dex (Dalvik Executable) or .odex (Optimized Dalvik Executable) file formats

Unlike the Java Virtual Machine (JVM), which is stack-based, the Dalvik VM is a register-based virtual machine

This architectural difference allows for more efficient execution on devices with limited CPU and memory resources, which is ideal for mobile environments.

Dalvik was the default runtime environment in Android versions prior to API level 21 (Lollipop)

It was eventually replaced by the Android Runtime (ART), which was introduced as a preview in Android 4.4 (KitKat) and became the default in Android 5.0

ART maintains compatibility by using the same .dex bytecode format as Dalvik, but differs significantly in its execution model

While Dalvik used Just-in-Time (JIT) compilation, ART initially used Ahead-of-Time (AOT) compilation — compiling bytecode into native machine code at install time, resulting in faster startup and improved performance

In later Android versions, ART evolved to include hybrid JIT + AOT and Profile-Guided Optimizations (PGO), further enhancing runtime efficiency and battery performance.