If you want to use a separate preconditioner, you need to call PETScKrylovSolver.set_operators(A, P) where P is your preconditioner matrix. I don’t think there’s an interface to that through the generic solve() syntactic sugar.
Like I’ve said in the past, solve() is a gross oversimplification which detracts from the underlying operations that need to be performed.
I strongly recommend designing your own Newton solver as above, or use a PETScSNESSolver, or use PETSc directly through petsc4py.