What is a Virtual Machine
A virtual machine is a software construct that acts as a container for installing and running conventional operating systems on a server hardware managed by a hypervisor. It is an isolation boundary between the operating systems running on the shared hardware.
An operating system running on a virtual machine is completely unaware of the fact that it is indeed running on a virtual machine and resources assigned to it are also shared among other virtual machines. It assumes ownership of every resource that is assigned to it. Managing the sharing of resources among virtual machines is the duty of the hypervisor. The performance of the virtual machine is dependent on the hypervisor's ability to manage the shared resources.
When a virtual machine is created, it is assigned resources such as the CPU, memory, network interface, and storage. These resources are slices from a larger pool of resources that the server hardware can provide.