I recently had to take a look at the possibilities of running Sharepoint 2013 on virtual servers for a customer.
I had do find some information and luckily Microsoft has a good set of articles about the topic.
Here are some of the articles I used to familiarizes myself with Sharepoint 2013 running in a virtual environment.
The way virtualization works, if you use the same hardware specification for a physical server as a virtual server, the physical server will always outperform the virtual one.
This means it will require more hardware e.g. more RAM and more CPU cores to use virtualized servers. So depending on the environment and number of servers you have to make a judgment of cost vs. flexibility etc.
In our case we ended up with a recommendation of running Microsoft SQL server on a dedicated physical server and running Sharepoint 2013 services and front-end servers on virtual machines.
For general information on Microsoft and virtualization see the following links.