From Fedora Project Wiki

Kernel and kexec

Kexec is a fastboot mechanism that allows booting a Linux kernel from the context of an already running kernel without going through BIOS. BIOS can be very time consuming especially on big servers with many peripheral connections. This saves time for the developers who boot a machine numerous times.

How to Use Kexec (i386, x86_64, ppc64)

1. Preload the kernel you want to boot into. Following is a sample command line, with \ as a line-break for printing purposes; this should appear as one line:

kexec -l /boot/vmlinuz-2.6.15-1.1955_FC5smp \
--initrd=/boot/initrd-2.6.15-1.1955_FC5smp \
--command-line="ro root=/dev/VolGroup00/LogVol100 rhgb"
Command line can be obtained with the command cat /proc/cmdline'

2. Reboot the machine. It boots into a new kernel without going through the BIOS: