Not quite sure what are the differences between these terms on VPS. Here is my understanding which might be terribly wrong. So please feel free to correct me.
For example:
128MB guaranteed with 256MB Burst:
the max physical RAM you can actual use is 256MB, although it depends on whether the host node has enough free RAM to allocate the other 128MB.
128MB guaranteed with 128MB VSwap:
although the VSwap shows under "swap" in "free -m", actually it is still in the physical RAM of the host server. the container is just artificially slowed down to emulate the "swap" effect.
128MB guaranteed with 128MB swap:
the "real" swap is a portion of the hard drive, will only be used when your physical RAM is not enough.
So my understanding is:
if you are using 100MB RAM, everything is in physical RAM in all 3 cases, so performance-wise, Burst = VSwap = Swap
if you are using 200MB RAM, Burst (everything still in physical RAM) > VSwap (everything in physical RAM but slowed down) > Swap (start to rely on disk I/O). Not sure if this is correct because I have no idea to which degree the VSwap will be slowed down. But I guess VSwap should still be better than "real" swap which get HDD involved.
Then what's the advantage for VSwap over Burst?