Choosing a network adapter depends on the VMware ESXi version number and the guest operating system running on the virtual machine. This guide describes the different network adapter options available for VMware ESXi virtual machines.
Available Network Adapters
Only those network adapters that are appropriate for the virtual machine you are creating are available configuration options in the Choose Networks screen.
Vlance: This is an emulated version of the AMD 79C970 PCnet32- LANCE NIC, and it is 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.
VMXNET: The VMXNET virtual network adapter has no physical counterpart. VMXNET is optimized for performance in a virtual machine. 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.
Flexible: The Flexible network adapter 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. With VMware Tools installed, the VMXNET driver changes the Vlance adapter to the higher performance VMXNET adapter.
E1000: An emulated version of the Intel 82545EM Gigabit Ethernet NIC. A driver for this NIC is not included with all guest operating systems. Typically Linux versions 2.4.19 and later, Windows XP Professional x64 Edition and later, and Windows Server 2003 (32-bit) and later include the E1000 driver.
Note: E1000 does not support jumbo frames prior to ESXi/ESX 4.1.
E1000e: This feature emulates a newer model of Intel Gigabit NIC (number 82574) in the virtual hardware. This is known as the "e1000e" vNIC. e1000e is available only on hardware version 8 (and newer) virtual machines in vSphere 5. It is the default vNIC for Windows 8 and newer (Windows) guest operating systems. For Linux guests, e1000e is not available from the UI (e1000, flexible vmxnet, enhanced vmxnet, and vmxnet3 are available for Linux).
VMXNET 2 (Enhanced): The VMXNET 2 adapter is based on the VMXNET adapter but provides some high-performance features commonly used on modern networks, such as jumbo frames and hardware offloads. This virtual network adapter is available only for some guest operating systems on ESXi/ESX 3.5 and later. 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 2 network adapter available.
VMXNET 2 is supported only for a limited set of guest operating systems.
VMXNET 3: The VMXNET 3 adapter is the next generation of a paravirtualized NIC designed for performance, and is not related to VMXNET or VMXNET 2. It offers all the features available in VMXNET 2, and adds several new features like multiqueue support (also known as Receive Side Scaling in Windows), IPv6 offloads, and MSI/MSI-X interrupt delivery.
VMXNET 3 is supported only for virtual machines version 7 and later, with a limited set of guest operating systems.
Migrating virtual machines that use enhanced VMXNET
VMXNET 2 was introduced with ESX 3.5. Virtual machines configured to have VMXNET 2 adapters cannot migrate to earlier ESX hosts, even though virtual machines can usually migrate freely between ESX 3.0 and ESX 3.0.x.
If you must migrate a virtual machine between later and earlier hosts, do not choose VMXNET 2.
Upgrading from ESX 2.x to ESX 3.x
When a virtual hardware upgrade operation transforms a virtual machine created on an ESX 2.x host to an ESX 3.x host, Vlance adapters are automatically upgraded to Flexible. In contrast, VMXNET adapters are not upgraded automatically because most Linux guest operating system versions do not reliably preserve network settings when a network adapter is replaced. Since the guest operating system thinks a Flexible adapter is still Vlance, it retains the settings in that case. If the upgrade replace a VMXNET adapter with a Flexible adapter, the guest operating system erroneously discards the settings.
After the virtual hardware upgrade, the network adapter is still VMXNET, without the fall back compatibility of the Flexible adapter. Just as on the original earlier host, if VMware Tools is uninstalled on the virtual machine, it cannot access its network adapters.
Adding virtual disks
Adding an existing earlier (ESX 2.x) virtual disk to an ESX 3.x virtual machine results in a de facto downgrade of that virtual machine to ESX 2.x. If you are using ESX 3.x features, such as enhanced VMXNET or Flexible network adapters, the virtual machine becomes inconsistent. When you add an existing ESX 2.x virtual disk to an ESX 3.x machine, immediately use the Upgrade Virtual Hardware command to restore the virtual machine to the ESX 3 version. This problem does not arise when you add earlier virtual disks to an ESXi/ESX 4.0 virtual machine.
Note: Executing the Upgrade Virtual Hardware command changes the ESX 2 virtual disk so that it is no longer usable on an ESX 2 virtual machine. Consider making a copy of the disk before you upgrade one of the two copies to ESX 3 format.
The most of the information of the article taken from VMware Knowledge Base