SD stands for "Secure Digital", SDHC stands for "Secure Digital High Capacity", SHXC stands for "Secure Digital eXtended Capacity". From the names only you can tell he main difference in all three i.e. storage capacity. SDHC cards offers between 4GB to 32GB and SDXC offers more than 32GB.
Can SDXC slot read SDHC?
Answer: The SDXC slot can be found in many Macs. The slot allows your computer to read the SD, SDXC, and SDHC cards that are often found in digital cameras. This allows you to simply remove the SD card from your camera and put it in the SDXC slot of your computer rather than hook your camera up to the computer.
What is the difference between SD SDHC SDXC memory cards?
"Standard" SD cards max out at 2GB capacity, based on their classification and the controller used by SD-only devices. Most SD cards you'll find today are technically SDHC, with capacities between 4GB and 32GB. The largest class is SDXC, or Secure Digital Extended Capacity, can range from 64GB to 2TB.