  So, if you have a DC running as a virtual machine, and it is handing out DHCP and managing DNS, imagine that you accidentally reboot the server one day, it reboots on its own, or it experiences some type of hardware malfunction. Let's say you have 30 people working in the office, only 5 of whom need to access the services provided by the virtual machines, the others use web-based tools for their job. What happens is that instead of 5 people not able to do their jobs because the server is down, the other 25 can't do their jobs either, because they 1) lost their network address, 2) have no DNS, and 3) cannot authenticate to anything else on the network because there is no DC to authenticate to. Having a separate physical DC will allow you to maintain DHCP and DNS in the event that the virtual DC is unavailable.   Between VMware and Hyper-V which one will work much reasonable for this type of solution? Do they run on top of OS or do they run as OS?   They ARE the OS. Choosing between them is completely dependent on choice, experience, environment, and cost. Hyper-V comes included in Windows Server. There is a free stand-alone version, but if you are familiar with Windows Servers and their management, this might make more sense. Otherwise, the downfall is the purchase of a Windows Server license. VMware is free, but Support is not. If you are not prepared to learn it first, you will want to consider purchasing a support contract. 
Hi I want to start implementing virtual servers (virtual datacenter) for my company but don't have an idea where to start as I've never worked with VMs before. This will be a total new infrastructure of about 5 - 7 servers Thank you Kakaza
Virtualisation Advise