Kernel services are routines that provide the runtime kernel environment to programs executing in kernel mode. Kernel extensions call kernel services, which resemble library routines. In contrast, application programs call library routines.
Callers of kernel services execute in kernel mode. They therefore share with the kernel the responsibility for ensuring that system integrity is not compromised.
"System Calls Available to Kernel Extensions" lists the system calls that kernel extensions are allowed to use.
Following are the categories of kernel services:
Programming in the Kernel Environment Overview.
Block I/O Buffer Cache Kernel Services: Overview.