import bpy,bmesh

ob = bpy.data.objects['Cube']
mesh=bmesh.from_edit_mesh(bpy.context.object.data)
for v in mesh.verts:
    v.select = True

# trigger viewport update
bpy.context.scene.objects.active = bpy.context.scene.objects.active

Docs

import bpy,bmesh

ob   = bpy.data.objects['Cube']
mesh = bmesh.from_edit_mesh(ob.data)
for v in mesh.verts:
    v.select = True

# trigger viewport update
bpy.context.scene.objects.active = bpy.context.scene.objects.active

Docs