LinuxTechi - https://www.linuxtechi.com/enable-nested-virtualization-kvm-centos-7-rhel-7/
Nested virtualization means to configure virtualization environment inside a virtual machine. In other words we can say nested virtualization is a feature in the hypervisor which allows us to install & run a virtual machine inside a virtual server via hardware acceleration from the hypervisor (host).
In this article, we will discuss how to enable nested virtualization in KVM on CentOS 7 / RHEL 7. I am assuming you have already configured KVM hypervisor. In case you have not familiar on how to install and configure KVM hypervisor, then refer the following article