There are a few different methods you can use to move the VMs over. I am going to assume that you will be deploying VMware on new hardware. These methods would be a little different if you were going to format your XenServer hosts and install VMware ESX on them as you wouldn't have both systems running at once.
Method #1: VMware Converter - The VMware converter is a utility available from VMware. It will install an agent on your XenServer hosted VM. It will then clone the VM to a new VM on the ESX host. Next, it will update the new VM to contain the VMware Tools software. Lastly, it will shutdown the old VM and then power on the new VM ensuring you don't get duplicate name/IP warnings. On a positive side, the original VM is relatively unchanged and you can revert to it quickly by shutting down the new VM. On a negative side, it takes a long time and has a high failure rate.
Method #2: Export/Import an OVF - In XenCenter, you can export any VM as an OVF file. OVFs are generic VM containers that can then easily be imported into VMware. The positive side is simplicity. The negative side is that the VM is completely unchanged. You typically want to uninstall the XenServer Tools from a VM before installing the VMware Tools for ESX. Unfortunately, you can only do that when the VM is running on a XenServer host. So before you export, clone the VM. Then, uninstall the XenServer tools on the clone. Finally, export the clone as an OVF and import that to ESX.
Method #3: Native Guest OS Backup/Restore - This is the most time consuming method, but it is also the most reliable. What you do is take a backup of your VM using the native OS tools (like Windows Server Backup, for example). Then, you create a new VM in VMware ESX and restore your backup to the server. This leaves the original VM untouched, and ensures a clean copy of the guest on the VMware ESX server.
Let me know if there is any other information I can provide,