The Trusted Computing Base (TCB) is defined as the total combination of protection mechanisms within a computer system. The TCB includes hardware, software, and firmware. These are part of the TCB because the system is sure that these components will enforce the security policy and not violate it.
The security kernel is made up of hardware, software, and firmware components at fall within the TCB and implements and enforces the reference monitor concept.
Reference: AIOv4 Security Models and Architecture pgs 268, 273