It is possible.
Two ways I've used in the past with a Samsung TV.
CEC is a communication method over the HDMI cable and can be used to send commands like power on/off, volume up/down etc to the TV
Infra Red: Use an IR LED and LIRC to send a command to turn on your TV. Disadvantage with this approach is that most manufacturers only have one code for on and off meaning that if your TV is already off and the Pi tries to turn it off again, it will actually turn on.