Hybrid kernel

For performance reasons, however, in version 4.0 and later, these modules (which are often implemented in user mode even on monolithic systems, especially those designed without internal graphics support) run as a kernel-mode subsystem.

Applications access system services by calling into the OS personality DLLs mapped into their address spaces, which in turn call into the NT run-time library (ntdll.dll), also mapped into the process address space.

The NT run-time library services these requests by trapping into kernel mode to either call kernel-mode Executive routines or make Local Procedure Calls (LPCs) to the appropriate user-mode subsystem server processes, which in turn use the NT API to communicate with application processes, the kernel-mode subsystems and each other.

[8] Apple uses a heavily modified OSFMK 7.3 functioning as a hybrid kernel with parts of FreeBSD included.

Like some other modern kernels, XNU is a hybrid, containing features of both monolithic and microkernels, attempting to make the best use of both technologies, such as the message passing capability of microkernels enabling greater modularity[citation needed] and larger portions of the OS to benefit from protected memory,[citation needed] as well as retaining the speed of monolithic kernels for certain critical tasks.

Structure of monolithic kernel, microkernel and hybrid kernel-based operating systems
The Windows NT operating system family's architecture consists of two layers ( user mode and kernel mode ), with many different modules within both of these layers.
The XNU Kernel