A microkernel is a minimal system kernel providing only basic operating system services (system calls), while other services (commonly provided by kernels) are provided by user-space programs called servers. Microkernels provide services such as address space management, thread management, and IPC, but not netoworking or display for example.
Advantages of the microkernel approach to system design: (a) adding new service does not require modifying the kernel, (b) it is more secure as more operations are done in user mode than in kernel mode, (c) a simpler kernel design and functionality typically results in a more reliable operating system.
A monolithic kernel is a kernel architecture where the entire kernel is run in kernel space in supervisor mode. In common with other architectures, the kernel defines a high-level virtual interface over computer hardware, with a set of primitives or system calls to implement operating system services such as process management, concurrency, and memory management in one or more modules.
Even if every module servicing these operations is separate from the whole, the code integration is very tight and difficult to do correctly, and, since all the modules run in the same address space, a bug in one module can bring down the whole system. However, when the implementation is complete and trustworthy, the tight internal integration of components allows the low-level features of the underlying system to be effectively utilized, making a good monolithic kernel highly efficient. In a monolithic kernel, all the systems such as the filesystem management run in an area called the kernel mode.
Source: Wikipedia.org
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