Virtual machine network adaptor types.

When you create or configure a virtual machine, you can specify network adapter (NICs) type.

The type of network adapters that are available depend on the following factors:

■	The virtual machine version.
■	Whether the virtual machine has been updated to the latest version for the current host.
■	The guest operating system.

The following NIC types are supported:

Vlance:

Emulated version of the AMD 79C970 PCnet32 LANCE NIC, an older 10 Mbps NIC with drivers available in most 32-bit guest operating systems except Windows Vista and later. 

A virtual machine configured with this network adapter can use its network immediately.

Flexible:

Identifies itself as a Vlance adapter when a virtual machine boots, but initializes itself and functions as either a Vlance or a VMXNET adapter, depending on which driver initializes it. 

Without VMware tools installed, it runs in Vlance mode. However, when you install VMware tools, it changes to high performance VMXNET adapter.

E1000:

Emulated version of the Intel 82545EM Gigabit Ethernet NIC, with drivers available in most newer guest operating systems, including Windows XP and later and Linux versions 2.4.19 and later.

VMXNET:

Optimized for performance in a virtual machine and has no physical counterpart. Because operating system vendors do not provide built-in drivers for this card, you must install VMware Tools to have a driver for the VMXNET network adapter available. 

Enhanced VMXNET (VMXNET 2):

Based on the VMXNET adapter but provides high-performance features commonly used on modern networks, such as jumbo frames and hardware offloads. VMXNET 2 (Enhanced) is available only for some guest operating systems on ESX/ESXi 3.5 and later.

VMXNET 3:

Next generation of a paravirtualized NIC designed for performance. VMXNET 3 offers all the features available in VMXNET 2 and adds several new features, such as multiqueue support (also known as Receive Side Scaling in Windows), IPv6 offloads, and MSI/MSI-X interrupt delivery. VMXNET 3 is not related to VMXNET or VMXNET 2.

SR-IOV (Single Root I/O Virtualization):

vSphere 5.1 and later supports Single Root I/O Virtualization (SR-IOV). SR-IOV is a specification that allows a single Peripheral Component Interconnect Express (PCIe) physical device under a single root port to appear to be multiple separate physical devices to the hypervisor or the guest operating system. 

SR-IOV uses physical functions (PFs) and virtual functions (VFs) to manage global functions for the SR-IOV devices. 

PFs are full PCIe functions that include the SR-IOV Extended Capability which is used to configure and manage the SR-IOV functionality. It is possible to configure or control PCIe devices using PFs, and the PF has full ability to move data in and out of the device. 

VFs are lightweight PCIe functions that contain all the resources necessary for data movement but have a carefully minimized set of configuration resources. 

SR-IOV-enabled PCIe devices present multiple instances of themselves to the guest OS instance and hypervisor. The number of virtual functions presented depends on the device. For SR-IOV-enabled PCIe devices to function, you must have the appropriate BIOS and hardware support, as well as SR-IOV support in the guest driver or hypervisor instance.

SR-IOV Architecture: 

Image: VMware

You should choose this adapter type if you are running latency sensitive workload and need high performance adapter. However, do check if your guest operating system does support this type of adapter as it is supported by limited versions of operating systems. 

Also, number of available  virtualization features like vMotion, DRS, FT and other will be reduced as they are not compatible with SR-IOV. Please check VMware KB article on SR-IOV for information. 

You can also check VMware KB article on which adapter you should choose for virtual machine.