What is the difference between the Linux Kernel and the Android Kernel? What are the modifications done in linux kernel to make it an android kernel ? By this question I mean what are the differences between linux kernel and android kernel ?
The differences change from version to version (both of Linux and of Android), and the exact kernel is different for each device. A kernel for Android is a mainstream Linux kernel, with additional drivers for the specific device, and other additional functionality, such as enhanced power management or faster graphics support.

Many features in the Android kernel are later added to upstream Linux after the open-source community has accepted (and perhaps modified) them, but others are never upstreamed, either because they're not acceptable to the Linux kernel developers, or because they're only applicable to Android or to specific hardware.