> It was hacked only using JTAG (hardware debug interface), not via software hacks.
That's just not true. Please, read the links.
The "King Kong exploit" utilized the ability to read/write arbitrary system memory using shaders on the GPU. This is done by modifying the unsigned shaders on a King Kong game demo. It does require that you modify the firmware on your 360's DVD-drive to be able to run a burned disc, but it's just a SATA drive and modifying firmware involves plugging into a PC and running an updater.
From there, it's purely software to exploit a software hole in the hypervisor's system call interface and gain full access.