http://ibmsystemsmag.blogs.com/aixchange/lpar/

Assuming I have my facts right (I borrowed them from the presentation, please correct me in Comments if you disagree with the information) VMware ESX 3.5 allows for four virtual CPUs per VM, 64 GB per VM, 192 VMs on a server, 32 CPU threads on a server and 256 GB on a server. ESX 4.0 allows for eight virtual CPUs per VM, 255 GB per VM, 320 VMs on a server, 64 threads on a server, and 1024GB on a server. PowerVM allows for 256 virtual CPUs per VM, 8192 GB memory per VM, 1000 VMs on a server, 1024 threads per server, and 8192 GB on a server.

Let’s look at flexibility once your VM is running. PowerVM virtualization allows you to make dynamic
changes to virtual CPUs, memory and I/O devices, and have integrated LPAR and WPAR support with PowerVM.

None of this is possible with ESX 3.5. With ESX 4.0, you can add but not remove virtual CPU, add but not remove memory. You can only make some dynamic I/O device changes, and limited direct access to I/O devices. The same arguments can be made with OracleVM Server for SPARC or HP Integrity VM 4.0. Oracle/Sun allows for Sun Logical Domains on UltraSPARC T1/T2 servers only--they allow for 32 partitions on a T1 or 128 on a T2. You can add or remove CPU, but only add virtual I/O. You can perform warm migrations with constraints. There's no support for dedicated I/O. With HP you can have 8 CPUs max, 64 GB ram. To do dynamic logical partitioning you need to reboot your LPAR. There's no support for dedicated I/O. There's no dynamic CPU sharing.