A kernel debugger is a debugger present in some operating system kernels to ease debugging and kernel development by the kernel developers. A kernel debugger might be a stub implementing low-level operations, with a full-blown debugger such as gdb, running on another machine, sending commands to the stub over a serial line or a network connection, or it might provide a command line that can be used directly on the machine being debugged.
Operating systems and operating system kernels that contain a kernel debugger:
The Windows NT family includes a kernel debugger named KD, which can act as a local debugger with limited capabilities (reading and writing kernel memory, but not setting breakpoints) and can attach to a remote machine over a serial line, IEEE 1394 connection, USB 2.0 or USB 3.0 connection. The WinDbg GUI debugger can also be used to debug kernels on local and remote machines.
BeOS
DragonFly BSD
Linux kernel; No kernel debugger was included in the mainline Linux tree prior to version 2.6.26-rc1 because Linus Torvalds didn't want a kernel debugger in the kernel.KDB (local)
KGDB (remote)
MDB (local/remote)
NetBSD (DDB for local, KGDB for remote)
OS X, Darwin which runs the XNU kernel using the Mach component
