Most of the "live streaming" channels for PlayOn are plugins or scripts. www.playonscripts.com/ and www.playonplugins.com/
Considering that PlayOn is transcoding on-the-fly, it's no surprise that it's SD, and also many of the sources are SD or less.
USATVNOW is a live streaming service for US Citizens abroad. As such, it will not work in the USA unless you go through a proxy server. There is a basic free service and a paid service. For the paid plans what you’ll get is a long list of channels; ABC, CBS, CW, FOX, NBC, PBS, My9*, A&E, Animal Planet, Bravo, Cartoon Network, CNBC, CNN, Comedy Central, Discovery Channel, ESPN, Food Network, FX, History, Lifetime, National Geographic, Nickelodeon, Spike, Syfy, TBS, TNT, USA. *All "LOCAL" channels are Pennsylvania, and are included in the "FREE" service.
1.) Add the channel at https://owner.roku.com/add/USTVNOW
2.) Go to miniprox.com (this is a proxy to sign-up for the service)
3.) Type ustvnow.com in the "Enter URL" box
4.) Then create an account.
5.) Check your email and confirm the account.
6.) Go to the USTVNow on the Roku and get your linking code
7.) Next link the Roku at USTVNOW
It looks like by adding some developer scripts like Superusvox ($5) or UTLive($2) you can watch HD content (when it's available) through Playon and XBMC. So for me by having Playon with those scripts added running on our main PC all the other computers in the house can run XBMC and watch internet TV. $57- not too bad.