Desktop Environment is not neccessary for Server usage, though. But Sometimes installation or using an application requires Desktop Environment, then build Desktop Environment as follwos.
[1] Install KDE Desktop Environment on here. [root@~]# yum -y groups install "KDE Plasma Workspaces"
[2] Input a command like below after finishing installation of new packages. [root@ ~]# echo "exec startkde" >> ~/.xinitrc
[root@ ~]# startx
[3] KDE Desktop Environment starts like follows.