It's bizarre that the computer magazines I read (and obsessively reread) would not have run an article like this.
EMS was pageable. You could had up to four 16 KB pages below 1MB that were fully accessible, but you had to select which address above 1MB each of those four pages had to point to.
So in short, XMS was inefficient, because you had to copy, and EMS was more efficient because you simply decided which part of the extended memory you wanted to map to lower memory.
I think, EMS was emulated in real mode on the 80386 chip by using its virtual 8086 mode. This was not possible on the 80286, so the 80286 could only use XMS in real mode.
I briefly developed using both EMS and XMS, and as I recall, XMS was somehow less painful to get up and running and generally work with, but the performance conscience part of me obviously favored EMS's paging mechanism.
Fortunately, 32-bit memory came along quickly both in the form of Windows and DOS protected mode.
When the 386 came out, it was common to have more than 1MB on the motherboard, but the motherboard (typically?) didn't natively emulate the EMS bank switching scheme, probably because at about that time, everyone figured that people were going to stop using DOS and switch to OS/2, and be running 32 bit code that could natively address higher than 1MB.
When that didn't happen right away, Quarterdeck came out with QEMM/386, which used the Virtual 8086 features of the 386 to emulate EMS in software. Microsoft then shipped a not quite as good clone of QEMM as EMM386 with DOS 5.0.
The "Megamemory Explained" graphic used in this post comes from the Jan. 14, 1986 issue of PC Magazine, part of an article called "Enlarging the Dimensions of Memory." It doesn't cover XMS as the 386 was so new as to be effectively nonexistent at the time, but otherwise seems to be a pretty thorough explanation.
I was an Apple guy and a late comer to the PC world, and PC Magazine was pretty much a must-read back in the day. Lots of good technical content.