Unlike an ammeter, a voltmeter must have a very high resistance. In theory, no current should flow through it. If current flows, then a potential difference is set up across the voltmeter. If the voltmeter were connected in series, it would take the lion's share of the available emf in a loop, according to Kirchoff's Laws so voltmeters are in parallel. Even then, unless the voltmeter's resistance is very high it will reduce the current flowing through the component being measured and thus lower the measured potential difference.
