I learn from Google Search that "shift+ins" works. I have tried that with Ubuntu 14.04 (Unity) and Debian (lxde). It works, but not for all cases. Now, I am using a workaround below instead:
xsel | xclip -selection c
It passes the text I select to clipboard and then I can use ctrl+v to paste instead. The bad thing is that it trashes whatever is stored in the clipboard (ctrl+c). But it is reliable. Feel free to map it with other tools at your convenience.