Mac OS X is based on the XNU kernel, a microkernel Mach kernel with a BSD userland, which makes Mac OS X's kernel a hybrid-kernel.
Android is based on the Linux Kernel. A kernel is a program that handles I/O and basically all the hardware stuff. So Android OS is device specific. (However there are some linux distributions which are device specific and you have to compile the kernel yourself.) That's why you can't install KitKat for Sony on an HTC device. You have to make it (compile) to suit your device's hardware specs.
kernel is everything in unix os
The kernel and the shell are two different parts of the OS or operating system. The shell interprets the commands from input. The kernel houses the heart of the OS like the command files and execute applications.
The OS makes calls to the system kernel (although sometimes the kernel is bundled with the OS). Applications make their calls to the OS, which then makes calls to the kernel.
Mac OS X is built on the XNU (X is Not Unix) kernel which is a hybrid combining elements of the Mach kernel and FreeBSD.
Mac OS X uses a Mach kernel and FreeBSD utilities at it's core.
the kernel is the part of the OS that interfaces with the hardware
shell, kernel
The Mac OS X Kernel is a hybrid based on the XNU kernel derived from the former NextStep implementations. Much of the code is open source and freely available. (See links below)
The kernel is the central control program of Unix and the majority of other operating systems.
These are separate and unrelated tasks. To check the version of the running kernel in Linux, use the command uname -r. To upgrade the kernel, either use your distro's package manager (if any) to update the system, or download and compile the Linux kernel source.