Diferences between x86 and x86_64 DVD?

Hi, anyone knows why I can boot the x86 DVD without problem, green
screen, install, everything ok, but the x86_64 version ask me to select
legacy isolinux boot (only option) and then nothing, I mean, what’s the
diference between them, x86 uses a linux default kernel to boot, x86_64
uses a linux-xen to boot… why?