The free hyperv server is just windows standard core (command line only, no gui).
If you are learning hyper v I would install standard server with the gui (desktop experience) . There negligible performance different between gui and core installations. In a production environment windows standard core is the best option, simply because there is less there for a hacker or virus to attack. As long as you are using Windows standard exclusively for its hyperv role you can install it without using one of your licences. This also will allow you to use features not available in the free hyper v server, provided that you can justify using them for the hyper v server roll.