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.
An OS reside in HDD, SSD, flash drive or CD/DVD. When you boot the computer the kernel is copied to RAM. Now other parts of OS will be loaded to RAM as and when required.
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.
This is done via a technique known as a loadable driver; the actual hardware driver is loaded into the system dynamically. Since devices can be loaded and unloaded they do not have to be compiled into the system 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
modprob
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)