Up until now, the configurations that we've described, by and large, have specified the domU's boot configuration in the config file, using the
kernel, ramdisk, and
extra lines. However, there is an alternative method, which specifies a
bootloader line in the config file and in turn uses that to load a kernel from the domU's filesystem.
The bootloader most commonly used is PyGRUB, or Python GRUB. The best way to explain PyGRUB is probably to step back and examine the program it's based on, GRUB, the GRand Unified Bootloader. GRUB itself is a traditional bootloader—a program that sits in a location on the hard drive where the BIOS can load and execute it, which then itself loads and executes a kernel.
PyGRUB, therefore, ...