It depends on your virtualisation solution and setup. If done correctly, it will have little difference compared to a physical machine. If done incorrectly, it will result in slow-downs. There are just too many possibilities.
If your storage-pool uses images residing on top of an existing file-system, it will be less efficient as compared to a raw partition or even fiber-channel storage. If your VM defaults to a 10Mbps NIC, it may slow things down. If you use OS-level virtualisation, it can help reduce memory requirements because the kernel and libraries can be shared across VMs.
You need to understand your requirements and study the different virtualisation platforms available and architecture best-practices used.