VirtualBox has an extremely modular design with well-defined internal programming interfaces and a client/server design. Targeted at server, desktop and embedded use, it is now the only professional-quality virtualization solution that is also Open Source Software. These won't work for VirtualBox, but I see that there are various virtualbox-guest-* packages in the repositories, but only for VirtualBox 5.2.18.VirtualBox is a powerful x86 and AMD64/Intel64 virtualization product for enterprise as well as home use. I was able to work around the problem by installing the open-vm-tools and open-vm-tools-desktop packages on the KDE Neon guest. The Player version is old, the KDE Neon is too new. However, with KDE Neon, I could not get VmWare's guest additions installed. But that goes end-of-life shortly, and I decided on KDE Neon as its replacement for all my computers.

I was previously using Linux Mint 17.3 and that worked fine and I had no difficulty getting it to work. I do have a KDE Neon guest on a work computer (Windows 7 host), but running VmWare Player. I never had any issues installing the guest additions with VirtualBox. I have various guests (Linux and Windows), but none with KDE Neon on VirtualBox (KDE Neon is my host and using the latest VirtualBox 6.0.4). But installing these may be problematic on newer Linux guests. However, for this to work, you need to install the Guest Additions. Settings are Disabled (default), Host To Guest, Guest To Host, and Bidirectional.

Sharing the clipboard is controlled by the Devices -> Shared Clipboard VirtualBox VM menu option. I'm not sure what you mean by 'KDE breaks in guest'.

Virtualbox extension pack 6.04