An Xen-based system is started through the GRUB2 bootloader
by using (for example) something like:

        echo    'Loading Xen xen ...'
        multiboot2        /boot/xen.gz placeholder
        echo    'Loading Linux 5.4.0-72-lowlatency ...'
        module2           /boot/vmlinuz-5.4.0-72-lowlatency placeholder root=... ro quiet splash
        echo    'Loading initial ramdisk ...'
        module2 --nounzip /boot/initrd.img-5.4.0-72-lowlatency

Notice that:
- The "xen.gz" executable is used as a kernel
- The real kernel (vmlinuz-5.4.0-72-lowlatency) and its
  initramfs (initrd.img-5.4.0-72-lowlatency) are loaded
  as modules
  - Remember? A module is some kind of "binary blob"
    loaded by the bootloader (integrated in qemu, in
    this case) without parsing it

The "xen.gz" executable will read the first module (the
"vmlinuz-..." Linux kernel) from memory, parse it (as a
compressed Linux kernel) and relocate it in memory, so
that it can be executed.

The Linux kernel is then started and finds its compressed
initramfs in memory, using it to populate the initial
ramdisk filesystem (by uncompressing and extracting the
CPIO archive).