Yes, XEM and NEM are the same thing; the main difference to note here is that XEM is the ticker symbol for NEM. What this means is that on the cryptocurrency markets, or on your preferred broker/exchange you will never see NEM/BTC listed, you will always see XEM/(Any asset that it is paired to). Similarly to how Bitcoin's ticker symbol is BTC, NEM's ticker symbol is XEM. XEM and NEM are the same thing, just names used in 2 different instances.