KDE Software Compilation is the set of frameworks, workspaces, and applications produced by KDE to create a beautiful, functional and free desktop computing environment for Linux and similar operating systems. It consists of a large number of individual applications and a desktop workspace as a shell to run these applications.
Make sure you installed archlinux (minimal install) with Internet Connection.
Then Type these command to install Full KDE 4 Desktop.
$sudo pacman -S kde
For Minimal KDE install
$sudo pacman -S kdebase
KDE using kdm login manager by default. After installation of KDE,
$sudo systemctl enable kdm
For login manager you can try another like slim or lxdm. :)
That's it, then reboot.
Choose KDE on your login manager, then login. :)
Here's my Archlinux with KDE.. :D