In WFS geometry is also just a property. If you do not use PropertyName parameter then all the parameters are included but if you use PropertyName then you must include into the request all the (non-mandatory) parameters that you want .
Compare
http://demo.geo-solutions.it/geoserver/wfs?service=WFS&version=2.0.0&request=GetFeature&TypeNames=states&count=2&PropertyName=STATE_NAME&outputformat=application/json
and
http://demo.geo-solutions.it/geoserver/wfs?service=WFS&version=2.0.0&request=GetFeature&TypeNames=states&count=2&PropertyName=STATE_NAME,the_geom&outputformat=application/json
The name of the geometry property is checked with the DescribeFeatureType request
http://demo.geo-solutions.it/geoserver/wfs?service=WFS&version=2.0.0&request=DescribeFeatureType&TypeName=states
You can see below that for this feature type the name of the geometry property is "the_geom". You can also see that all properties are nillable=true and minOccurs="0" so they are not included in the result set if they are not requested. If some properties in a WFS service are not-nillable or if they have minOccurs greater than zero then they are mandatory and always included in the result even if not requested with PropertyName.
<?xml version="1.0" encoding="UTF-8"?><xsd:schema xmlns:gml="http://www.opengis.net/gml/3.2" xmlns:topp="http://www.openplans.org/topp" xmlns:wfs="http://www.opengis.net/wfs/2.0" xmlns:xsd="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" targetNamespace="http://www.openplans.org/topp">
<xsd:import namespace="http://www.opengis.net/gml/3.2" schemaLocation="http://demo.geo-solutions.it:80/geoserver/schemas/gml/3.2.1/gml.xsd"/>
<xsd:complexType name="statesType">
<xsd:complexContent>
<xsd:extension base="gml:AbstractFeatureType">
<xsd:sequence>
<xsd:element maxOccurs="1" minOccurs="0" name="the_geom" nillable="true" type="gml:MultiSurfacePropertyType"/>
<xsd:element maxOccurs="1" minOccurs="0" name="STATE_NAME" nillable="true" type="xsd:string"/>
<xsd:element maxOccurs="1" minOccurs="0" name="STATE_FIPS" nillable="true" type="xsd:string"/>
<xsd:element maxOccurs="1" minOccurs="0" name="SUB_REGION" nillable="true" type="xsd:string"/>
<xsd:element maxOccurs="1" minOccurs="0" name="STATE_ABBR" nillable="true" type="xsd:string"/>
<xsd:element maxOccurs="1" minOccurs="0" name="LAND_KM" nillable="true" type="xsd:double"/>
<xsd:element maxOccurs="1" minOccurs="0" name="WATER_KM" nillable="true" type="xsd:double"/>
<xsd:element maxOccurs="1" minOccurs="0" name="PERSONS" nillable="true" type="xsd:double"/>
<xsd:element maxOccurs="1" minOccurs="0" name="FAMILIES" nillable="true" type="xsd:double"/>
<xsd:element maxOccurs="1" minOccurs="0" name="HOUSHOLD" nillable="true" type="xsd:double"/>
<xsd:element maxOccurs="1" minOccurs="0" name="MALE" nillable="true" type="xsd:double"/>
<xsd:element maxOccurs="1" minOccurs="0" name="FEMALE" nillable="true" type="xsd:double"/>
<xsd:element maxOccurs="1" minOccurs="0" name="WORKERS" nillable="true" type="xsd:double"/>
<xsd:element maxOccurs="1" minOccurs="0" name="DRVALONE" nillable="true" type="xsd:double"/>
<xsd:element maxOccurs="1" minOccurs="0" name="CARPOOL" nillable="true" type="xsd:double"/>
<xsd:element maxOccurs="1" minOccurs="0" name="PUBTRANS" nillable="true" type="xsd:double"/>
<xsd:element maxOccurs="1" minOccurs="0" name="EMPLOYED" nillable="true" type="xsd:double"/>
<xsd:element maxOccurs="1" minOccurs="0" name="UNEMPLOY" nillable="true" type="xsd:double"/>
<xsd:element maxOccurs="1" minOccurs="0" name="SERVICE" nillable="true" type="xsd:double"/>
<xsd:element maxOccurs="1" minOccurs="0" name="MANUAL" nillable="true" type="xsd:double"/>
<xsd:element maxOccurs="1" minOccurs="0" name="P_MALE" nillable="true" type="xsd:double"/>
<xsd:element maxOccurs="1" minOccurs="0" name="P_FEMALE" nillable="true" type="xsd:double"/>
<xsd:element maxOccurs="1" minOccurs="0" name="SAMP_POP" nillable="true" type="xsd:double"/>
</xsd:sequence>
</xsd:extension>
</xsd:complexContent>
</xsd:complexType>
<xsd:element name="states" substitutionGroup="gml:AbstractFeature" type="topp:statesType"/>
</xsd:schema>