The Linux kernel includes support for the kexec subsystem which provides a means for a running Linux kernel to chain load into another kernel.
It can be used to test another kernel without a complete reboot, but it is most commonly used as part of the kdump kernel crash dumping framework to capture a kernel crash dump and save logs. When the kernel crashes (especially in Enterprise Linux distros), it can be configured to automatically execute a special kdump kernel that has been previously loaded into a reserved region of memory.