By default the TeamCity comes with NUnit plugin. But if you have the Unit Test project built with xUnit framework you will have to install xUnit plugin for TeamCity.
In the runner type while adding steps there will not be any xUnit option by default.
Let's follow the steps for installing the the xUnit plugin.
After following the above steps you will notice that the xUnit plugin will be enabled.
You will be able to see th xUnit type in the Runner type while using it in the Build Steps of your configuration.