copy/paste from ubuntu/centos7 xterms back to windows applications does not work

[Regentrude]

I run VirtualBox 5.2.8 r121009 on a windows 10 machine (all updates installed).
I have several linux virtual machines running e.g. Centos 7 (KDE desktop), Ubuntu LTS 16.2.
All of them have the guest edition installed.

I can perfectly fine select any text in a windows application and paste it into a konsole or an xterm or any text editor in Ubuntu or Centos 7. The text shows up perfectly.
Now the other way around is a different story. If I use a konsole window on either linux distro and use Control-Shift-C to copy the selected text, I can paste it in Windows in anywhere I want and it shows up. e.g. in a web browser, as text in a text editor or OneNote. No problems here.
Now I select text in an xterm, by just pressing down the left mouse button and moving over the text to select, or double click on a word or tripple click to get the line.
If I now want to paste that selection on the windows anywhere, does not show up. It pastes what ever was in the clipboard from my last copy/paste started in windows or the last copy/paste started in a console in Linux.
Why is copy in xterm to paste into Windows not working, when copy in linux console and paste in windows application is working just fine?
What am I missing?
Thanks for the help.