Here's the story as I see it:
There are two factors: hardware and software
DOUBLE DENSITY
The standard IBM controller provided in an IBM XT only supports double density. That means 5.25"-360K and 3.5"-720k
If you have the 9May86 ROM BIOS in your XT, then it will happily read/write both drives.
If you have an earlier ROM BIOS, to read/write a 3.5"-720k, you need to add a suitable DRIVER.SYS line in CONFIG.SYS
HIGH DENSITY
First, you will need to have a floppy controller that supports high density (from a hardware perspective).
Most third party floppy controllers that I've seen contain a BIOS extention and a set of DIP switches.
If you have one of those, you just set the switches and off you go.
If your floppy controller isn't of that type, you should be okay if you have the 9May86 ROM BIOS.
If you have an earlier ROM BIOS, you need to add a suitable DRIVER.SYS line in CONFIG.SYS