Hello,
The basic difference is that on Poweredge servers, servers ending in "xd" will have more HDD slots.

I can share the following list where you can see the comparison about R740 and R740xd https://dell.to/3mu5UYr

detailed explanation for R740 https://dell.to/2WEPnGR

detailed explanation for R740xd https://dell.to/2J1aQq1

XC models come with integrated Nutanix software. You can review the detailed description as follows, https://dell.to/2KznSf0, again under this link, you can check the differences of XC servers under the comparison heading.

detailed explanation for XC740xd series https://dell.to/3gYU2MY

Let us know if this helps!