Edit in JSFiddle

function go() {
    var photosLyr = new ol.layer.GeoportalWMTS({
                layer: "ORTHOIMAGERY.ORTHOPHOTOS"
            }) ;
    var map = new ol.Map({
        target: 'map',
        layers: [
            photosLyr
        ],
        view: new ol.View({
            center: [288074.8449901076, 6247982.515792289],
            zoom: 15
        })
    });    
    var azi = new ol.control.MeasureAzimuth({
    });       
    map.addControl(azi);
    var area = new ol.control.MeasureArea({
    });
    map.addControl(area);
    var length = new ol.control.MeasureLength({
    });
    map.addControl(length);
    var elep = new ol.control.ElevationPath({
    });
    map.addControl(elep);
    var draw = new ol.control.Drawing({
    collapsed: false}) ;
    map.addControl(draw);
    var ls = new ol.control.LayerSwitcher(
    {options: {
      collapsed: false
    }}) ;
    map.addControl(ls) ;
    
    // 1) Save last drawn features
    $('#savelastkml').click(function(){
        try{
            var kml = draw.exportFeatures();
            if(kml){
              // on enregistre le KML
              $('#kmlsaved').val(kml);
              $('#savelastkml').prop( "disabled", true );
              $('#loadlastkml').prop( "disabled", false );
              map.getLayers().forEach(function(layer) {
                  //console.log(layer);
                  // on supprime toutes les couches, sauf celle initiale
                  if(layer.gpLayerId != 0)
                      map.removeLayer(layer);
               });
            } else {
              alert("Merci d'ajouter une ou plusieurs annotations avant de sauvegarder.");
            }
        } catch (err) {
          alert(err);
        }
    });
    
    // 2) Load last drawn features
    $('#loadlastkml').click(function(){
     		$('#loadlastkml').prop( "disabled", true );
        $('#savelastkml').prop( "disabled", false );
        
    		var vectorSource = new ol.source.Vector({
          opacity: 1,
          features : new ol.Collection(),
          visible: true
        });

			// From : http://gis.stackexchange.com/questions/219023/openlayers3-add-a-source-vector-from-string-not-url-for-layers
      var features = new ol.format.KMLExtended().readFeatures(
        $('#kmlsaved').val(),
        {
          dataProjection: 'EPSG:4326',
          featureProjection: map.getView().getProjection()
        });
      vectorSource.addFeatures(features);

      var vector = new ol.layer.Vector({
        title: 'Données chargées',
        source: vectorSource,
        visible: true,
      });

			// on remet à jour les composants (suggéré par @gcebelieu)
     	draw.setMap(map);
      map.addLayer(vector);
			draw.setLayer(vector);
			ls.addLayer(
          vector,
          {
            title: 'Données chargées'
          }
        );
    });
}

Gp.Services.getConfig({
    apiKey: "jhyvi0fgmnuxvfv0zjzorvdn",
    onSuccess: go
});

var infoDiv = document.getElementById("info");
infoDiv.innerHTML = "<p> Extension OL3 version " + Gp.ol3extVersion + " (" + Gp.ol3extDate + ")</p>";

<input type="button" id="savelastkml" value='Save last KML'/>
<input type="button" id="loadlastkml" value='Load last KML' disabled=disabled/>
<input type="hidden" id="kmlsaved" name="kmlsaved"/>
<div id="map"></div>
<div id="info"></div>