Firecracker supports booting x86 kernels in "PVH direct boot" mode as specified by the Xen project. If a kernel is provided which contains the XEN_ELFNOTE_PHYS32_ENTRY ELF Note then this boot mode will be used. This boot mode was designed for virtualized environments which load the kernel directly, and is simpler than the "Linux boot" mode which is designed to be launched from a legacy boot loader.
PVH boot mode can be enabled for Linux by setting CONFIG_PVH=y in the kernel
configuration. (This is not the default setting.)
PVH boot mode is enabled by default in FreeBSD, which has support for Firecracker starting with FreeBSD 14.0. Instructions on building a FreeBSD kernel and root filesystem are available here.