HttpContext.Current.Request.Url.Host

or

SiteDefinition.Current.SiteUrl.Host

You can use the ISiteDefinitionRespository to get access to all of the defined hosts. 

 protected SiteDefinition GetSiteDefinitionFromSiteUri(Uri sitemapSiteUri)
        {
            return this.SiteDefinitionRepository
                .List()
                .FirstOrDefault(siteDef => siteDef.SiteUrl == sitemapSiteUri || siteDef.Hosts.Any(hostDef => hostDef.Name.Equals(sitemapSiteUri.Host, StringComparison.InvariantCultureIgnoreCase)));
        }

If you're not doing anything really custom, have you looked at using Geta.SEO.Sitemaps package? It generates SEO sitemaps on a schedule for configured hosts and works well in multi-site scenarios. Could save you some time.