The Xorg project provides an free software implementation of the X Window System – the foundation for a graphical user interface. Desktop environments such as E17, GNOME, KDE, LXDE, and Xfce provide a complete graphical environment. Various window managers offer alternative and novel environments, and may be used standalone to conserve system resources. Display managers provide a graphical login prompt.

In Parabola you find the locally installed desktops in /usr/share/xsessions so you can put those lines as "exec ..." (e.g. startkde or dwm) into ~/.xinitrc