Among the entire supported virtual network adapter types, VMXNET is the paravirtualized device driver for virtual networking. The VMXNET driver implements an idealized network interface that passes through the network traffic from the VM to the physical cards with minimal overhead. The three versions of VMXNET are VMXNET, VMXNET 2 (enhanced VMXNET), and VMXNET 3.

The VMXNET driver improves performance through a number of optimizations, as follows:

• It shares a ring buffer between the VM and VMkernel and uses zero-copy, which in turn saves CPU cycles. Zero-copy improves performance by having the VMs and VMkernel share a buffer, reducing the internal copy operations between buffers to free up CPU cycles.
• It takes advantage of transmission packet coalescing to reduce address space switching.
• It batches packets and issues a single interrupt rather than multiple interrupts. This improves efficiency, but in some cases, with slow packet-sending rates, it could hurt the throughput while waiting to get enough packets to send them.
• It offloads the TCP checksum calculation to the network hardware rather than use the CPU resources of the VM monitor. Use vmxnet3 or the most recent model if you can. Use VMware Tools where possible. For certain unusual types of network traffic, sometimes the best model isn't optimal; if you have poor network performance, experiment with other types of vNICs to see which performs best.