Google has just released some very detailed data on Android growth rates by version and which versions are running on which kinds of devices. The data reflects devices running the latest Google Play Store app, which is compatible with Android 2.2 and higher, and the data is captured by measuring the devices that visited the Google Play Store in the prior 7 days. Among other things, it shows that the latest KitKat Android version has grown its market share significantly. Month over month, adoption of KitKat is up by 37 percent.
Here is a snapshot of Google's breakdown of Android versions in terms of market share: