ESX vs ESXi

ESX and ESXi official comparison:
http://kb.vmware.com/kb/1006543 - VMware ESX and ESXi 3.5 Comparison
http://kb.vmware.com/kb/1015000 - VMware ESX and ESXi 4.0 Comparison

Probably the big difference is that ESXi has a POSIX Management Appliance that runs within the vmkernel and ESX has a GNU/Linux Management Appliance that runs within a VM.

ESX vs. ESXi which is better?
http://communities.vmware.com/blogs/vmmeup/2009/04/07/esx-vs-esxi-which-is-better
Dilemma: buy vSphere with ESXi4 or with ESX4?

ESXi vs. ESX: A comparison of features
http://blogs.vmware.com/esxi/2009/06/esxi-vs-esx-a-comparison-of-features.html

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ESXi limitations

• No official interactive console (there is only a "unsupported" hidden console: http://www.virtualizationadmin.com/articles-tutorials/vmware-esx-articles/general/how-to-access-the-vmware-esxi-hidden-console.html)
• No official supported SSH access (there is only a "unsupported" access: http://www.yellow-bricks.com/2008/08/10/howto-esxi-and-ssh/)
• No support for some 3th part backup programs (see also Backup solutions for VMware ESXi)
• No support for some 3th part programs
• No hot-add feature for virtual disk in ESXi 4.0, but only on ESXi 3.5 (or also on ESXi Advanced with hot-add license)
• No SNMP "get" polling"
• No virtual Serial or Parallel port
• No USB disk in the "console"
• No support for mount ext2/ext3/ntfs/cifs filesystem in the console
• No fully functional esxtop
• No fully functional esxcfg-mpath
• No vscsiStats tool
• No PXE install or scriptable installation (but seems that there is a solution: http://www.mikedipetrillo.com/mikedvirtualization/2008/11/howto-pxe-boot-esxi.html)
• Normally with a standalone ESXi host you'll get 1 hour of performance data, the graph should be set to reset every 20 seconds (you can also extend the performance data to 36 hours will a simple hack - http://www.vm-help.com/esx/esx3i/extending_performance_data.php)

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Free ESXi limitations

• ESXi 4.0 free can not be connected to a vCenter Center (it required a VC agent license)
• VCB does not work (it require a VCB license)
• Lot of 3th part backup programs does not work with free ESXi (see also: Backup solutions for VMware ESXi)
• RCLI and VMware Infrastructure toolkit are limited to read-only access for ESXi free
• The free version of VMware ESXi does not support SNMP
• The free version of VMware ESXi does not support Active Directory integration at this time
• Free ESXi EULA has some interesting restrictions including enforced read only mode for v4 and later versions of v3.5.

See also: What's the difference between free ESXi and licensed ESXi?
http://vmetc.com/2008/08/10/whats-the-difference-between-free-esxi-and-licensed-esxi/
VMware ESX/ESXi EULA - http://www.vmware.com/download/eula/esx_esxi_eula.html

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ESXi advantages

• Small footprint
• Can be installed on a flash card or USB key
• Does not use a vmdk for console filesystems (as ESX 4.0 does)

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Sources

Serious ESX vs ESXi in the Enterprise
Compare ESXi with ESX?
Re: list of features that you cannot use with ESX4i
Looking for more good comparisons of ESX 4 and 4i
Re: ESXi Free edition - limitations