Your Pi needs about 700mA to operate; to quote the Raspberry Pi user guide:
Connecting the Pi to the USB port on a desktop or laptop computer is possible, but not recommended. As with smaller chargers, the USB ports on a computer can't provide the power required for the Pi to work properly
The "smaller chargers" bit refers to the fact that many small smartphone micro-USB chargers can only supply 500mA, so the bottom line is that it depends on the mA available from the USB port on the TV.
I use a 700mA plug-in adapter, but you can use the output from a powered USB hub too, if it has sufficient power.