In computing, kexec (kernel execution) is a mechanism of the Linux kernel that allows "live" booting of a new kernel "over" the currently running kernel. kexec skips the bootloader stage (hardware initialization phase by the firmware or BIOS) and directly loads the new kernel into memory, where it starts executing immediately. This avoids the long times associated with a full reboot^[1], and can help systems to meet high-availability requirements by minimizing downtime.

While feasible, implementing a mechanism such as kexec raises two major challenges:

1. the new kernel will overwrite the memory of the currently running one, while it is still executing
2. the new kernel will usually expect all physical devices to be in a well-defined state (as they are after system reboot, when the BIOS (or firmware) resets them to a "sane" state). Bypassing a real reboot may leave devices in an unknown state, and the new kernel will have to recover from that.

현재 사용하는 커널을 그대로 둔 상태로, 그 위에서 새로운 커널로 라이브 부팅하는 리눅스 커널의 메커니즘이라고 하네요. 부트로더 단계를 건너뛰고 곧장 새로운 커널을 메모리에 로드하여 실행을 개시한다고 합니다. 풀 리붓을 할 필요를 줄여 다운타임을 최소화하는 효과가 있다고 하네요. (위키피디아)