Hello,

can a Xen/KVM VPS Node server support overselling/overcommitting of the RAM/CPU? Is there any information about this topic?

I found KVM supports it, but no details on how to achieve that:
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Virtualization_Administration_Guide/chap-Virtualization-Tips_and_tricks-Overcommitting_with_KVM.html

except Virtualizor way ($1 to $9 monthly, ipv6 support)

Small update: Here is described KVM overcommitting resources like RAM, CPU: http://serverascode.com/2013/02/20/overcommitting-with-kvm.html
https://pve.proxmox.com/wiki/Dynamic_Memory_Management
http://www.linux-kvm.org/page/KSM <- KSM = Kernel Samepage Merging
https://wiki.fd.io/view/VPP/How_To_Optimize_Performance_(System_Tuning)#KSM

Here is mentioned "KVM memory ballooning": http://www.linux-kvm.org/page/Projects/auto-ballooning
How to use virtio: http://www.linux-kvm.org/page/Virtio
"How to use Virtio: Get kvm version >= 60; Get Linux kernel with virtio drivers for the guest; Get Kernel >= 2.6.25 and activate ;
CONFIG_VIRTIO_PCI=y (Virtualization -> PCI driver for virtio devices)
CONFIG_VIRTIO_BALLOON=y (Virtualization -> Virtio balloon driver)
"
Another overview of the way to overcommit RAM on KVM: https://avikivity.blogspot.ro/2008/04/memory-overcommit-with-kvm.html (10 years old article)