beagleBoard Start up Problem

I have beagleboard XM revision A3 board. I have connected beagleboard with serial to usb converter to my pc. I am not giving any supply voltage. Will it work? In manual it is mentioned that it will take power from USB. But while connecting with USB, the power LED doesnt glow while using external power supply, power LED glows.Can i use both external power as well as usb ?