# yum group install 'Xfce Desktop'
installs all packages needed for Xfce.
But this does not change the currently used display manager (DM).
To get rid of kdm:
# systemctl disable kdm
I don't know exactly what the default DM of the Xfce-Spin is - but to configure - say - lightdm:
# yum install lightdm
# systemctl enable lightdm
The disable/enable systemd commands configure if those services are started during boot.
To actually start/stop the DM on a running system:
# systemctl stop kdm
# systemctl start lightdm
Although a reboot of Fedora is very fast, you can test the startup of services even faster like this:
# systemctl rescue
# systemctl default