VMware released a great whitepaper about virtual I/O architectures and performance impacts:
The storage subsystem is a critical determinant of system performance. The key to good storage performance is to identify factors and system configuration settings that affect performance and understand how to set these in order to achieve the best results.
The basic determinants of performance are the operating system, the data transfer size, and the access pattern. In the virtual machine environment, the drivers for the available virtual adapters are also a factor. VMware ESX Server virtual machines can use virtual BusLogic and virtual LSI Logic SCSI adapters. The default driver for a virtual machine depends on the guest operationg system. for example, Windows 2000 guests use the Microsoft-supplied BusLogic adapter by default, while Windows Server 2003 guests use the LSI Logic adapter by default.
This document provides a characterization of storage performance for a VMware ESX Server system with an EMC CX500 SAN as the storage back end. The goal is to provide performance data and system resource utilization at various load levels. Throughtput, I/O rate, and response time for various data sizes and access patterns provide sizing guidelines. This baseline data is expected to help debug performance problems and facilitate server consolidation for I/O intensive workloads.
Download it here.