Let M be a compact hyperkähler manifold with maximal holonomy (IHS). The group H 2 (M, R) is equipped with a quadratic form of signature (3, b 2 − 3), called Bogomolov-Beauville-Fujiki form. This form restricted to the rational Hodge lattice H 1,1 (M, Q) has signature (1, k). This gives a hyperbolic Riemannian metric on the projectivization H of the positive cone in H 1,1 (M, Q). Torelli theorem implies that the Hodge monodromy group Γ acts on H with finite covolume, giving a hyperbolic orbifold X = H/Γ . We show that there are finitely many geodesic hypersurfaces, which cut X into finitely many polyhedral pieces in such a way that each of these pieces is isometric to a quotient P(M )/ Aut(M ), where P(M ) is the projectivization of the ample cone of a birational model M of M, and Aut(M ) the group of its holomorphic automorphisms. This is used to prove the existence of nef isotropic line bundles on a hyperkähler birational model of a simple hyperkähler manifold of Picard number at least 5 and also illustrates the fact that an IHS manifold has only finitely many birational models up to isomorphism (cf.