DO NOT UPDATE TO ATV 5.0 or iOS 5.1 IF YOU WANT TO KEEP USING XBMC (no jailbreak)
#1
and it should go without saying, XBMC does not currently run on the ATV3 (again, no jailbreak). There's no way for us to know when it will, as we don't handle jailbreaks, so don't ask.

There are plenty of jailbreak websites out there that will cover this and answer jailbreak-specific questions.
#2
See: AppleTV 2 (black) Jailbreaking ATV2 Running 4.4.4 or Less, After Apple Stopped Signing Older Firmware

For help on jailbreaking an existing 4.4.4 ATV, even though Apple stopped signing for 4.4.4. This only helps people on 4.4.4 and lower to jailbreak, and will not help people with ATV OS 5.0 downgrade.

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DO NOT UPDATE TO ATV 5.0 or iOS 5.1 IF YOU WANT TO KEEP USING XBMC (no jailbreak)1