After upgrade from 14.04 LTS to 16.04 LTS plugins missing. In the menu with plugins appear only "KXML Hello World" Reproducible: Always
This would be a bug with Ubuntu, not KDE. Feel free to report it on Ubuntu's Launchpad.
See also https://bugs.launchpad.net/ubuntu/+source/gwenview/+bug/1594156 This is fixed in Kubuntu 17.04 version.