Beagle Boards I2C Lines working Voltage

what is the working voltage of the I2C lines on the beagloeboard-xm? 1.8v or 3.3v?
Is this is same for all the I/Os (GPIOs).
How can I configure a particular GPIO : In the board-config file(arch/arm/mach-omap2/board-omap3beagle.c) for Beagleboard-xm or device tree file?
Please advice , I am newbie to beaglebone and just got pumped up by booting it up with a linux distro.

Thanks in advance

1.8V. All I/O on the XM is 1.8V.