Assuming that Russian losses continue to mount and Putin's stated objective of "regime" change remains out of reach, I'm trying to see a way out that both Kiev and Putin might eventually accept.
All I can come up with is:
* Kiev agrees to recognise Russian occupation of Crimea and formally cede it to Russia.
* Putin agrees to immediately withdraw all forces back into Crimea and Russia.
* Kiev agrees to cede either the Donbar or Luhansk regions (but not both) either to Russia or as an "independent" republic.
* Russia recognises the legitimacy of the Kiev government and Ukraine (with its new, reduced borders) as a sovereign state, independent of Russia.
* Kiev reserves the right to apply to join the EU and/or NATO, but agrees not to do so for at least 10 years.
* Russia formally acknowledges the above right, given the 10-year caveat.
* Kiev reserves the right to continue bringing in defensive weapons and receive military training from the West.
* Russia agrees that it if ever invades Ukraine again, it's claim to Crimea and Donbar/Luhansk becomes defunct.
* Discontinuation of Western sanctions are not part of the agreement.
That's a lot for both parties to swallow, but then it (a) stops all the killing; and (b) gives some "wins" for both.